Clean the threads, you'll thank yourself later.

You don't have to use a thread brush, the best way is to use an old head bold, take a die grinder, and groove the threads along the length of the bolt in 3-4 lines, then thread them in, and unthread them.

The grooves catch all the grime left in the threads, then when you back them out, you can clean them, and thread back in.

Yeah, it takes a while to do, but it cleans all of the goop out the right way.