If you are tired of carrying around a pocket (or non pocket) tuner, get a rack tuner. Never think about where your tuner is again. Plus you can constantly tune during shows.
If you love your pocket or hand tuner, then I say the next most necessity type gear if you are happy with your sound is a power conditioner. I'm not saying they make you sound any better or anything magical like that...it's just nice to plug in your amp to it and other misc things that you may have along the way (wireless, tuner, whatever) and only have one plug to plug when you get onstage. Plus plugging anything to the wall with no such protection is just a no-no for me. Changing fuses in between songs is just not cool.
I know its great to leave one rack space open for that extra airflow, but a tuner right on top of it is of almost no consequence. Unless the tuner is 16" deep in the rack, most tuners are like about 5" at max? That doesn't create much stress on the amp. That gives you one more space at the very top for that power plug or a wireless. Yeah.
-T