All name-brand gear makers do extensive in-house testing to ensure that their stuff can stand up to the rigors of washing. Most seek something like a "20/80" rule, meaning that after 20 washings the item retains 80 percent of its original water repellency. And that's for waterproof-breathable gear; usually stuff that's polyurethane-coated for complete waterproofing is even tougher.
So I recommend washing everything on a fairly regular basis. If it fits in the washing machine, let 'er rip on regular cycle (a front-loader's ideal as an agitator may catch straps and the like). Dry on warm, or air-dry (one exception: never run a tent through the dryer as the coating on the waterproof sections can indeed peel off). If in doubt, simply dunk the item in a tub of warm water, or hose it off on the driveway and clean with a soapy sponge.
A few caveats to keep in mind: don't wash sleeping bags any more than necessary, particularly synthetic ones; wear long underwear or use bag liners to help keep them clean; and tents often can be dealt with by using a damp sponge. Of course, it's always wise to look for the care tag on an item, just to make sure you aren't doing something boneheaded. Or, you can always ring up the manufacturer to check.
But do keep your gear clean! It's happier, you're happier, everyone is happier.
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