It all depends on which kit you buy. The spacers for the different 2.5" lifts are different heights. The CCM 2.5" lift spacers are 1-3/8" thick, the Auto Springs spacers are 1-1/2" thick, and the Total Performance spacers are 1-3/4" thick. From what I have seen on here and been told the CCM spacers make the truck sit perfectly level. I had the Total Performance spacers on my 07 Sierra and I had to replace the stock blocks with 2-1/2" blocks for it to sit level. I have ordered the Total Performance spacers again for my 09 Silverado and I am going to replace the rear blocks again just because I want it to sit higher. If you want it to sit level by only adding the front spacers then I would get the 2.5" CCM spacers off of ebay.
I have seen several trucks with the 2" kits and the front is still a little lower than the rear.
I have seen several trucks with the 2" kits and the front is still a little lower than the rear.