you only need to reapply with replacing 'old' paste that's already set up. if you were upgrading to a new cooler, then you must clean the processor and add new paste. however, when doing a fresh install, it's best to test mount the cooler, then unmount it to make sure you're getting good even contact (the paste is spread out and none is oozing over). if you've got a good square patch, just remount it (which is what you did).
it's impossible for the paste to 'become uneven' if the heatsink is mounted properly. (there's only a extremely thin layer in place to begin with and its sole purpose is to fill any microscopic void between the processor and the heatsink.) make sure you tightened the heatsink in a diagonal fashion to ensure even contact.
unless you're using benchmarking software to load the cpu and carefully tracking temps while benching, a small change in temp from casual observance is nothing to worry about.