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 think that this is the original post.
If they complain about amounts, tell them to complain to me. It's best if sources try to exhaust their source allocations, even if it means giving posts higher amounts than is typical. If you have 150 source merit and you only see 3 merit-worthy posts in a month, then I'd rather you over-give each of them 50 merit than let the merit expire. That way there are more people capable of sending merit, and the "merit economy" is less top-down.
I personally agree with this idea. I think that active circulation of Merit is important for the activation of the forum and each local board.