It's perfectly safe right up next to your house and on your deck. Mine sits on my patio with the back right up to the wooden garage wall.
The wood you use to smoke goes inside a closed fire box that has very little chance of letting anything out. That fire box goes inside the smoker which is a large, insulated, stainless steel box. The only place a hot coal could possibly escape would be for it to somehow get out of the fire box (almost impossible) and fall just precisely into the drain hole in the bottom of the smoker. That drain hole is much smaller than a dime, so the coal would have to be even smaller. At that point the coal would fall into a stainless steel pan where it would promptly die.
Your bigger/real risk is that you will splatter stuff on your deck when you're putting food into your smoker or taking it out. I have stuff drip/fall on my patio all the time (rub, marinade, etc.) when putting stuff in and grease/juice drip when I'm taking stuff out. Fortunately, my patio is rock and not covered where it drips so it mostly cleans itself up. If I were on a deck I'd put down some kind of splatter mat whether that's a door mat, a piece of cardboard, or a fancy mat like sold on the Smokin-It site just to protect the look of the deck.
All of that said, you could actually get flame inside of the fire box. Sometimes the wood gets hot enough and enough oxygen gets into the smoker that it can ignite. You'll be able to tell post-smoke. If what you are left with is ashes then your wood caught on fire. If what you are left with is charcoal then it didn't. In any case, you're still perfectly safe... the fire box contains it and the relatively low oxygen environment inside the smoker keeps it from going crazy. There is ongoing debate about whether it impacts the flavor of the food if your wood catches fire. I've found that putting a double think piece of foil in the bottom of the firebox (between the wood and firebox) and poking smaller holes in it where the firebox holes exist will keep it from igniting 99.8% of the time.
