Coverage, coverage, coverage. Get that varnish on, as much as you can. That's my rule.

On horizontal surfaces, put it on as thick as possible.

On vertical surfaces, lay it on thick, almost to the point of sagging.

You have to be neat, or you'll spend extra time cleaning up runs. But, I'm not going to go nuts trying to get a mirror finish. I'd have to sand too much varnish off between coats.