For 4.0 point-buy, I also find a 20 (18 + racial) to be too expensive. There are just too many other good options and reasons to spread things around; 18 (16 + racial) just gives you a much more well-rounded character that it's worth it for me. This includes even wizards (in fact, wizards especially benefit from good dex, wis, con, and even the dreaded cha).
That said: I would never play a character with 17 or lower (after racial) as their primary stat, either. (Well, a dwarven fighter with a total 16 Str is not quite horrible, but I can't think of any other exceptions - and frankly there's still a good argument to be made that he'd be better off with a 17 or 18.)
And hilariously, from what I've read the devs seemed to have expected everyone to go for the 20 as well. (Even "suggested stats" in the character builder gives you a 20 in your primary.) So maybe my advice isn't that good. Then again, they also created (and then quickly fled from) 4 classes that semi-require two primary stats and it's impossible to get an 18 twice, so who knows.