Congrats on your good class! I also had a good one last night where we were able to tackle some tricky sequences. Regionals is in 2.5 weeks and I've not trialed since January. Eep!
As far as start lines go, yeah, in my experience it's not uncommon to have a dog itching to release and jumping the gun. It depends on distance, duration, level of arousal, etc.
Cohen used to have a killer start line stay. Now, it's complicated. The moment I place my hand on her collar to remove it, she starts barkbarkbarking. It takes a minute to get her attention back and her settled. She'll sometimes continue to bark on the start line and I wait for her to quiet before I release. The closest she comes to breaking her stay these days is standing up from the sit or down I leave her in as I take my lead out. I've been working on reinforcing good behaviour with food but I'm suspecting I'll see a regression in a trial atmosphere.
However, between runs I put her in a down-stay (off leash due to leash/collar removal issues) and she'll hold it indefinitely while I walk the course.
I used to have a perfect, calm, explosive start line and I took it for granted. Don't! Reinforce it! Also, I wouldn't really want to leave my dog lined up on a start line if she has to be there for "quite a few minutes". I may be sensitive to it because of my issues with Cohen, but I would want to have her hold a stay in another area so she knows that when she's lined up you're 100% connected with her and ready to play.