Once you're in the top 10 or 12 you're in a good spot. If your field is 20 riders you can move up much later because you're almost already there, even if you're at the back.
If you get into "good" position early then you're going to use a ton of energy defending it. All the riders behind you will try to eventually improve your position. Unless you're in a flat out 40 mph surge to the line you won't be able to hold your position simply by staying behind the rider in front of you. If I was feeling super strong I'd be up there at 2-3 to go (and spend 3-4 laps getting up there), if I'm not feeling great I'll make one bigger effort and move up with 1-2 to go.
Here's a training race where I was conservative with my field position even with 1 km to go, there was a group of riders dangling off the front, yet things worked out:
When it's single file it's usually not a great time to move up. If you insist on moving up then you can usually find a gap to slip into or you can use a corner to let the rider in front come to you (so in a left turn you hold your line, the rider in front goes wide hopefully, ends up in front of you, and the rider behind him now has to ride next to you or behind you).