It doesn't rain enough here for me to bother with it.
There are a few ways to deal with it.
One is, to put the type of tips on it where you don't have to worry about it, like BullHorns, or turnouts. Another, you could put "caps" on the stacks, or if you will be parking it alot with a high probability of rain, just temporary caps on the stacks.
If geting soot all over your truck isn't a big deal to you, then don't bother with anything,, you will need to have 12" of rain before you would run into problems.
If you want to let water drain out, then you can drill a 1/8" hole at the lowest part of your exhaust system and that will let the water drain out, but I just can't stand the "hole in the exhaust" sound.
I've had stacks for about a year on my truck without any holes, and I haven't had any problems yet,, besides the occasional soot shower.
Merrick Cummings Jr