Thank you for your response.

I'm pushing orders from a shopping-cart to OE. For that I'm using Openerp .netConnector with POCO entities.

Customer (incoice-contact, shipping-contact),
Order,
and Orderlines can be created. So now I must fine-tuning the taxes. Taxes are by product-line, but from shopping-cart perspective they are triggered on rules (invoice-country, shipping-country, EU, Extra-EU etc.) So the tax is coming from shopping-cart.When ignoring all this I can force a default tax (default class for now from OE = id 1 @ 15%). So I want to save this tax id with each sales.order.line  but I don't know how to handle this. I see that the table sale_order_tax contents this ids for each orderline as many as tax-classes.

Here is part of the order.item:

                   var neworderline = new OSalesOrderLine();
                   
                    //neworderline.productid = 19;// not uses until products are mapped
                    neworderline.taxId = ????
                    neworderline.orderId = orderId;
                    neworderline.name = item.Product.Name;
                    neworderline.productUomQty = Convert.ToDouble(item.Quantity);
                    neworderline.priceunit = Convert.ToDouble(item.PriceExclTax * currRate);
                    oeService.AddEntity<OSalesOrderLine>(neworderline);

Here is the propreties maping for this model:

[OpenErpMap("sale.order.line")]
    public class OSalesOrderLine : BaseNopModel
    {
        
            [OpenErpMap("name")]
            public string name
            {
                get;
                set;
            }
            [OpenErpMap("order_id")]
            public int orderId
            {
                get;
                set;
            }
            [OpenErpMap("price_unit")]
            public double priceunit
            {
                get;
                set;
            }
            [OpenErpMap("price_subtotal")]
            public double priceSubTotal
            {
                get;
                set;
            }
            [OpenErpMap("product_uom_qty")]
            public double productUomQty
            {
                get;
                set;
            }
            [OpenErpMap("tax_id")]
            public int[] taxId
            {
                get;
                set;
            }
            [OpenErpMap("id")]
            public int id { get; set; }
            [OpenErpMap("product_id")]
            public int productid { get; set; }
    }

I'm adding here the sniffed XML from actual code

<?xml version="1.0"?>
<methodCall>
  <methodName>execute</methodName>
  <params>
    <param>
      <value>
        <string>database</string>
      </value>
    </param>
    <param>
      <value>
        <i4>1</i4>
      </value>
    </param>
    <param>
      <value>
        <string>password</string>
      </value>
    </param>
    <param>
      <value>
        <string>sale.order.line</string>
      </value>
    </param>
    <param>
      <value>
        <string>create</string>
      </value>
    </param>
    <param>
      <value>
        <struct>
          <member>
            <name>name</name>
            <value>
              <string>product name</string>
            </value>
          </member>
          <member>
            <name>order_id</name>
            <value>
              <i4>65</i4>
            </value>
          </member>
          <member>
            <name>price_unit</name>
            <value>
              <double>1285</double>
            </value>
          </member>
          <member>
            <name>price_subtotal</name>
            <value>
              <double>0</double>
            </value>
          </member>
          <member>
            <name>product_uom_qty</name>
            <value>
              <double>2</double>
            </value>
          </member>
          <member>
            <name>product_id</name>
            <value>
              <i4>19</i4>
            </value>
          </member>
          <member>
            <name>tax_id</name>
            <value>
              <struct>
                <member>
                  <name>Item1</name>
                  <value>
                    <i4>6</i4>
                  </value>
                </member>
                <member>
                  <name>Item2</name>
                  <value>
                    <i4>0</i4>
                  </value>
                </member>
                <member>
                  <name>Item3</name>
                  <value>
                    <array>
                      <data>
                        <value>
                          <i4>1</i4>
                        </value>
                      </data>
                    </array>
                  </value>
                </member>
              </struct>
            </value>
          </member>
        </struct>
      </value>
    </param>
  </params>
</methodCall>

No clue about your current implementation, but if you're using the (XML/JSON)RPC connection you might want to call an on_change method on the object.

Because taxes are configured per product, when creating the sale order line the taxes are automatically provided upon selecting the product. So, calling on_change_product_id (or something similair) should provide you with the right taxes based on the product. 

Otherwise, when not using the product, you can go for the method of input using a tuple of 2 defined settings and the ids. This will result in a value like this: [(6,0,my_tax_ids)]. Again, this is from the top of my head, so you need to verify in code if this works.

In general: I hope you are not working outside of the ORM methods. It is highly inadvisable to do so. You will miss certain object workflows like that.