In my opinion you can't combine boom and lav in the same scene at all, it's a HUGE difference in character between 'em. The lavs have a much more "flat" sound, and where you might very well match up two different boom-mics to appear consistent by filtering, you can't really do that between lav and boom as the change in flatness and liveliness gets very obvious when hearing them both at the same time.
I rarely work with lavs at all, I don't like them and my employers mostly prefer the shine I can give by using high quality booming, but when I do it's mostly only in extreme wideshots without any closeups, or in long takes with extensive steadycam movement where there are no room to move smoothly.
Sometimes, though, you might get a project or two where there are no budget for ADR, not enough preparations, and no respect for wild-lines. In this case, having a clear take though it will sounds pretty weird in the mix (still ONLY meaning when jigsawed together, everything else is a matter of taste and need), compared to a rich, living sound where you have absolutely no idea what was said, is well worth it! Though the average Joe might deem it amateurish depending on how obvious the transition is, they will buy it. An unintelligible line, however, is not as easily forgiven and might ruin the whole take.
But mostly, if it's that bad, you discard the boom altogether and go with the lavs for that scene.
I was about to compare mixing lavs and boom-mics with Roger Rabbit, but that's very unfair of me. Instead, a great comparison is actually to try combine 35mm film with DV! It can be used either as an effect or by keeping 'em apart with good editing and planning, but it will never be the same, and must be treated carefully.