Hi Kenny,
I always teach a new guy to only use one number, so he doesn't get confused. Each lathe works differently. On my lathe I can engage the half nut on any line on my dial. On some lathes if you are cutting an even numbered thread, you can use any even number on the dial. Every lathe is different. I know many experienced tool and die makers who will only use one number on the dial just to be safe. A plus to only using one number is that it gives the new operator a natural pause. He can recheck that he hasn't bumped any of the dials or that he did reset the cross slide back to zero.