Well, if I were you, I'd ignore Opera and make sure IE and Mozilla work as a first call.
As a representation (which I'd say ClioSport is pretty typical).
55.74% of visits are IE7
21.26% of visits are IE6
20.30% of visits are Mozilla/5.0
00.12% of vists are IE5
Opera 9.2 has 0.91% which is even lower than the Yahoo bot!

Opera 9.3 has 0.03%
Opera 9.5 has 0.02%
So, make sure IE6/7 and Mozilla work.. then look at others
Safari is a good one to test against, as it is very standards based.. but you'll find it displays different to IE/Mozilla a lot!
Is annoying tho
