I guess it comes down to a few main options depending on situation.

1. Is the accountant an "official" odoo accountant, they may have their own account they can tag to your database.
2. You are a zero trust type person (nothing wrong with that), but you need to set them up an account of their own restricted to functionality they need.
3. You have a second user already (eg. for Point of Sale), or your admin account if you are crazy that you can share with the accountant for use. (no specific accountant extra account)
4. Your accountant comes on site, uses your account but has no access unless physically at your logged in machine. (no extra account)
5. Whatever your accountant needs you run the reports and just send them the data (no extra account)

The Triangle here is Trust, Time and Cost. 

If you pay more it takes less time and trust isn't an issue.

If you spend more time, you "save" the cost and trust isn't an issue.

If you trust your accountant more, you save on cost and time (but you may always wonder about security)