Don't count on it. Not nesc. because of the field (that may be changed accordingly) but more for the business logic. A lot of processes and links in the background keep track of wheter or not a res.partner record has a parent record. Also code on the sale and purchase module checks this.
If you only want an indication and no business logic, perhaps it would be best to not give the contact a parent at all. Then create a new relational field (one2many) to the res.partner table and you can freely fill in the relation without interfering with standard behaviour.
By the way: Out of curiosity I would like to know why you would need this? I cannot think of a situation where any of my contacts would be working for 2 or more different companies?