I normally run as a non-admin user. I won't go into the reasons why that's a good idea (they've been elaborated elsewhere); I'll limit myself to saying that I do think it's a good idea and something we should encourage other users to do. At the very least, we should not be encouraging people to create new admin accounts without strong reason.
A test account is for testing things. Almost by definition, if you're testing, you don't know what's going to happen. Using an admin account for testing is like swallowing the potion to see if it's poisonous.
If what you're testing is whether something behaves differently for admin/non-admin accounts, you can do that by temporarily making your test account an admin. But you should only do that if the admin-nature of the user is an essential feature of the test, and you should set it back after running that test.
I actually keep three test accounts lying around. One is a normal user, one uses file vault, and one has parental controls. There is also Leopard's Guest account. None of these has admin privileges.
Don't overlook Leopard's guest account. Most quick tests can be done right there, with no fuss or bother. In fact, the fact that the account wipes itself after every use makes it ideal for testing, because it precludes interactions between the test you're doing today and the test you did last week, whose details you've probably already forgotten.
