I believe that 16 in a primary stat, after racial modifiers, can work pretty well. It's not the optimal choice, but it works rather well. It is, I think, expected for characters whose racial modifiers and class primary ability don't synergise well (like, say, dwarf fighters).
I think going for an 18 in such cases (barring certain builds that can take it) is worse than settling for a 16 in primary attribute and distributing the rest among other abilities so as to have well-rounded defences.
I played a dwarf wizard with a starting Int of 16 for five levels (I'd have played him longer, but I moved away from my home town and haven't had the opportunity to play since), and while it is true that he missed more times than he hit, even a 20 Int wouldn't have helped him, since it was more due to my terribly low die rolls.
Regards.