Is there a received wisdom on when to use 1/4mill or 1/2mill charts?
It depends upon the speed that you fly, and to some extent how much room you have in the cockpit.
The object is not to map crawl, but to use the map to provide you with sufficient relevant information to navigate. As stated 1/2 Mil is the norm, helicopters may use 1/4 Mil and even 50K when looking for landing sites.