How to know when a dice throw is good
Reduce all stats to 10 - then look at the max skill points you can assign, 28 and above is ok. 30+ is good, 32+ is rare, more than 34 is likely never gonna happen unless you dice like 3 hours.
What does STR/00 mean...
Unmodified 18: +1 to hit, +2 to damage
18/01-18/50: +1 to hit, +3 to damage
18/51-18/75: +2 to hit, +3 to damage
18/76-18/90: +2 to hit, +4 to damage
18/91-18/99: +2 to hit, +5 to damage
18/00: +3 to hit, +6 to damage
(A 19 gives +3/+7.)
Why you should not bother throwing more than a 18 on strength:
If you have, in games like BG2 for example an permanent stat bonus +1 on STR. It will automatically raise your str to 19 (which means it actually gives you +5 STR.) That does not apply to Items, only to permanent stat increases or very very rare spell functions.
In BG2 TOB theres 2 such ways. But as for IWD i don't know.
But again, don't bother dicing 18/00 , theres no great point to it. To Hit spirals out of control anyway and +2 or +5 damage does not make any large difference lategame. As most Monsters where it would make a difference, are immune to critical hits.