Nope, if you put 2.5" in the front, your 4Runner will start to look like a rocketship. I've made mine level with a 4.5" lift in front and 3.5" in the rear ... basically, lifting the front either .5" or 1" higher than the rear will level it out. When I had 4" in the front and 2.5" in the rear, things measured pretty level, but due to the body shape, it looked like a rocket! And that was only 1.5" difference ...
Anyway, I definitely recommend that you only lift the front .5" - 1" higher than the rear.
If you do not plan on lifting the rear at all (as you mentioned only front spacers) then you will definitely want less than 2" ... unless you have the rear air leveling suspension (which supposedly levels itself out).
I hope this helps! Basically, people just do it to get the look they want, and despite what measurements say, the truck will NOT LOOK level with 2.5" in the front and nothing in the rear. It will appear to be sagging in the rear, severely.