I weigh and size my Mozzies. Looking in the tins I have, I feel no need to wash them and as I shoot spring don't lube them.
I had one bad tin where the pellets were shocking about a year ago but have since bought a sleeve and they are very good since.
I used to weigh to .1 of a grain but found half the tin was going in the plinking pot, so I now weigh from 7.75 to 7.95 and don't see any difference at all to my aim points.
Weighing sniffs out those rogue pellets that are extremely light or heavy. I think my record for bad mozzies is 7.53 and 8.15 grains, enough to miss those long targets(if it weren't hard enough already!!).
Sizing seems to reduce my groups a little but does reduce velocity in my 97 by about 15-20 fps over straight out the tin ones.
I have to set up my guns power to the mid 11's fpe with un-sized pellets then use sized pellets at a reduced velocity/power which is no big deal as my 97 seems better behaved in the high 10's to low 11's anyway!