I have always just installed them the way they were, then IF I notice that they may be getting a little wet (like the grease is coming out) I put a drop or two of oil on them. then the next time I am near them, I just replace them.
I buy my bearings in bulk when I can. thrust bearings are different, they are hard to find in bulk. but radial bearings can be found rather easily in bulk and pretty cheap. I usually buy them 20 at a time, that whey when I notice one is getting notchy, or I am even near one that is hard to get to, I go ahead and replace them.