Welcome to TTORA and I believe you can fit 4.5" and less on the backspacing. I say this because I sold my stock FJ wheels to a guy with an 07 Taco and they cleared the calipers by a long shot and I replaced them with 16x8 with 4.5" backspacing and they cleared the calipers on my FJ. Visit http://www.wheelersoffroad.com/ and/or give them a call-they're friendly, informative folk. The biggest concern you will likely have will be finding wheels that are compatible with the air pressure sensors that come stock on your rig. Search for that on the late model tacoma thread... I've seen a couple of recent posts.
Man the sports are a different breed when it comes to wheels. There is not a lot of 16x8's that don't rub the calipers w/out spacers. The FJ's don't rub, a lot of people like them on here. I went with the black steelies at wheelers. They fit without rubbing. My advice would be to look in the pics of taco's thread for the 05+. If you see something you like, pm the guy and get the specs from him and see what he had to do to make them fit. Be aware though, some rims that fit offroads, will rub on sports. it sucks, I know. But there are a ton of options to make these rims fit. Spacers, grinding, do some searching on those subjects. As far as backspacing, it has to do more with the rim design than backspacing. The FJ's are 4.5" spacing, one of the only few that I know with that bs that fit. I would say you are better w/ a 4" backspace. but even some 4" don't fit, they rub. Just have to do your research or go to wheelers, they work, and they are affordable.