It so happens that I was just this past hour out looking at some salmon redds, watching some pinks and coho spawning in a small tributary to the Snoqualmie. In extremely clear water less than a couple feet deep, standing on a high bank, watching the fish actively working their redds, with several "experts" on hand, the redds are pretty subtle and hard to spot. I can't imagine your average angler up to his crotch in cold water, concentrating on the timing of his double haul, is going to see more than he steps in, especially in a big pink year like this one. (And that includes myself, who's had the benefit of "expert" training.) Then again, it's not like you'll be crushing eggs with every other step, or that all things considered, the salmon's biggest problem is wading boots.
I'm not exactly sure where I am on this. I guess I'd have to agree with Sparse. If you're really worried about it, stay off the spawning gravel. And which is the spawning gravel you ask? That would be the gravel under the water. I don't know how else to "responsibly" answer that question. Whether that means you shouldn't fish in the fall, I honestly don't know.
The questions of whether redds can stand a little walking on, or whether our salmon populations can spare a redd or two, are not entirely black and white. It depends on the fish, their current status, and the river you're fishing in. So again, if you really care, take the time to educate yourself about the river you're fishing and the fish populations in that river. But if you're the better safe than sorry type, fish from the bank I guess.