Theymos prefers Merit sources to empty their source. He said (and I'm not going to find back the quote): if you find only a few posts worth meriting in a month, give them everything so they can redistribute sMerit again. I personally wouldn't do that, as dumping 50 Merit on a Newbie's post doesn't seem right to me.

A better forum is my incentive

My source went from 200 to 400 at the beginning of January, and was empty at the end of the month. I merited many posts at the beginning of January, so those sMerits were all replenished at once recently, and currently I have a hard time emptying my source again. On top of that, my "personal" sMerit pile is higher than ever.