It depends on how many components I'm putting on and how picky I feel about bent leads. If it's only a few components then I like to make it as clean as possible. However, most of the time I'm stuffing a bunch of components in all at once, so I put in a handful that are all the same height without bending the leads, use a small piece of paper or cardboard and flip the whole thing over, then solder the leads from there. Usually the weight of the board on the table is enough to hold everything in place.
Sometimes if I'm using helper hands, I'll actually use mini spring hair clips to hold that piece of cardboard on. This only works for small boards though. For bigger boards I resort to tacking or more often gently bending leads outwards. Actually, I just don't bend them all the way in the first place, rather I let the springiness of the leads hold the component in place. It's a bit trickier to get the angles just right, but I find it much faster and the lead angles aren't as severe.
