Last night I fixed a 6 slot Neo-Geo board for a forum member and this will probably blow your mind: The top board was really nasty so it got a scrub in the sink with a toothbrush and dish soap. I found that is the best method for thoroughly cleaning out the slots to remove the years of dust, grime, gunk, etc. that gets inside them.
The board was dried in the gas oven at 170 degrees then tested. After drying, every slot worked with no lines in graphics or garbled sound which is an indication of dirty slots.
I prefer that method of drying since it gets all the water out of the cracks and crevices. It's safe for most plastics: the only plastic I've ever had issues with is the square piece over the top of the memory card slot on the 2 slot Neo Geo boards. It will get soft and when you pull the board out simply lift the plastic back in place and blow on it and it'll set back where it needs to be. This type of plastic would really be bad off in a dishwasher if you use the heat dry.
Just don't do any washing or drying of boards that have batteries on them and you'll be fine. Batteries and water simply do not mix.
RJ