There are tons of guys, especially ones that work only in mono-steel (stainless or carbon) that grind after actual assembly (nailed together). The advantage of this, is that even if things aren't dead-flat/parallel before assembly, or even if you cant(tilt) the pivot or other pins during peening, you can grind everything "right". You can even grind a blade to land center, that otherwise wouldn't.
However, the limitation of this approach, is that it doesn't encourage you to make everything so that it'll come together with everything right in the first place, and it can severely limit your ability to utilize certain construction methods and materials. It makes having properly etched damascus components nearly impossible (yes, I do know of some ways people have figured out how to etch everything after it's already assembled and ground together, but it's tedious, and I doubt you can ever really be sure you didn't do internal damage with the acid). All that being said, there are some top end slipjoint makers that hammer everything together before grinding. Most of them (if not all) that I'm aware of however, do pretty specific configurations (usually, all stainless) of construction.
I personally build each component separately, completely finished, glue the handle material on, then pin it all together as the last step. This IMO gives the most versatility, and control over the final product.
Like Don, (and I make very small, thin, delicate slipjoints), I just hold the tang between my index finger and thumb, and rest the spine over the knuckles of my other index finger when grinding. I do this with full distal tapers, 1/10" or thinner blades, on a 3hp grinder running at 6,000 SFPM, and 50 grit belts, typically. I say all this, only to illustrate the point, that it's not that hard to do. Unlike Don, I typically grind my slipjoint blades 100% after heat treat. I do this primarily because a) I like to clamp my blades straight right out of the quench (even carbon), and b) I like to have plenty of room to clean-up decarb or adjust geometry after HT. The advantage here, in holding the blades with your fingers (once you're used to it enough to be safe) is that you always know when you're getting too hot and need to quench between grinding passes, which, the smaller, and thinner you get, the easier it is to burn a blade finish grinding, and ruin temper. However, this is really just a matter of preference.
I don't personally recommend doing your blade grinding loosely assembled with "temp pins", because you're liable to get something loosened up by grinding pressures, and have an accident frankly. You do however, often need to do profile refinement (grinding spine and handle belly) or handle work this way, especially if you're utilizing exotic materials where you need each component to be completely and finely finished before final assembly.
