I personally do not because I like to see how things fit me. I really like to see how an outfit looks in a mirror before I buy it. Also as a short male, not all clothes are created equally. Sometimes I'll try on a small shirt and the top will be fine but the entire shirt will just go down to near my knees. For those that do shop for clothes regularly online, what got you to make the decision?
I finally found that Duluth Trading Company has consistently-sized clothing, because they make most of it themselves. After one shipment where I ordered three different sizes, found the one that fit me, and returned the others, it's a no-brainer to order from them now. I know my size, and their clothing is consistent.
Almost every other retailer simply buys clothes from a variety of bulk manufacturers, and slap their label on it. So sizes are all over the board, since every mfg has their own idea of sizes.
So yeah. I buy from Duluth and Zappos, because I know the sizing is good. And when it isn't, it is dead-easy to return the stuff.