Whatever method you use, you need to practice with tiny amounts of bitcoin until you are satisfied you have a good system. Only then create a new wallet and move your other coins into it.
That's all well and good, but it's possible that only
1 in a million addresses generated by a tool will have problems, and so you could test 10 different addresses with tiny amounts and then be unlucky and get a broken address that you use with the rest of your coins.
The bug I linked to used to affect bitaddress. It doesn't any more, but we can't be sure that there aren't other such bugs, so be careful!