i started out wanting 20s too when i first began looking into purchasing a tacoma. after doing a LOT of research i have come to the following conclusions...
20s are just too big unless you are putting a 6" lift on it. thats the only way you could fit a tire with enough meet on it to keep from scratching your rims when wheeling. even then i think 20s wouldnt look right.
18s are a step in the right direction, but i think after doing more research into tire sizes you will find there arent a large amount of options.
17s have waaaay more options when it comes to different tire sizes and brands, and are well proportioned with the rest of the truck.
...just my .02