The two most useful signs in combat are of course Aard and Igni because they cause direct damage/effects, but depending on your approach you may want to use some of the others.

Normally you will want to favor Intelligence and Stamina if you want to become magic oriented, and you will want to increase Strength and Dexterity if you are sword oriented. So don't worry, choose whatever paths YOU like. As far as I know, you can't really 'break' your build unless you try really hard to do it on purpose.