2nd person when dealing with posts. really saves a huge amount of time. reduces amount of walking, over all. and when hand tamping holes. one person on one side, other person on other side. and just hand tamper back and forth between the 2 of you.
when it comes to drilling the holes. second person is there to help keep things straight, and then also help plumb the hole level better. but honestly. trying to keep all the holes nice and plumb and just be 1/2" off to a couple inches. can be rather difficult. a larger hole lets you have wiggle room. and able to adjust for those off angle drilling some, and get things nice and straight.
farm fence for around a field or pasture, were you could care less about fence looks. and just use various limbs from trees you cut down over the last year or so. is a different story.
found getting a level that bends into a L shape. and having a roll of duct tape on hand. so you can quickly tape / pull off level on each post. to level in both directions works pretty good. freeing your hands for tamping.
to note it, one area may be nice and easy to put posts in while other areas may be pain in rear to put posts in due to how much the dirt is compacted or what is in the dirt. and what it takes to drill or hammer a post in will and can vary a lot.
for me, i have had places were i had to bring more dirt in, to tamper in around a post. due to how loose the soil was to begin with. other places i have had more dirt from drilling a hole with PHD. and had to shovel some of the dirt away.
i like idea of using a post hammer to drive posts in myself as well, but some times it is the old ugly doing of more work that gets the final results, that look better and hold up better long term. some times not. note if using a post hammer. for wooden posts. extra time may be spent with a saw and pointing the ends of the post. so the posts are easier to drive.
if you have long streachs of straight fence, looking down the line can help get things straight. but you may want to run a string or cable to help keep things straight. when ya having to look around a post that just barely sticks out and blocks your view of trying to keep things straight.