I throw a lot of mine out , to be honest. even after washing you don't always get all the old chemical out.. and some times that might cause unwanted results (like if you had paint thinner on there and washed it and wanted to use it to, say, buff some wax out, and instead thinned the paint off).
In addition my washer specifically says to NEVER attempt to wash any clothing that has been soiled with any amount of flammable substance. I think thats just them watching their own ass, as we've all washed some pretty foul E36 M3 and hopefully none of us has burned down the washroom yet. But if you were washing a full load of fuel and brake cleaner soaked towels, well, youre pushing your luck.
Anytime I have to wash anything thats muddier or oiler than normal (which is fairly muddy) I take it to the laundromat. Pay a dollar or two and berkeley up their equipment. Its a lot easier.
Oh and btw if its REALLY dirty, adding a couple cups of ammonia usually does wonders towards getting it clean.