Good questions.
I think that telling the LARS controller you want to have a quick chat with the airfield is a great idea, as that controller might already have coordinated your track with the airfield, and save you the bother. In which case, your call on box2 would be a needless interruption to the zone controller.
What's not at all acceptable is when you have only one radio and you just go off freq to talk to someone else. When you have engaged (contracted) with the LARS controller, even for a Basic Service, you should let them know before changing.
Regardless, using Box 2 is a great idea when you have it, as you can still (try to) follow what's going on around you on the LARS freq.
There are differences of course, if the airfield is Linton or if it's Shobdon.