Clam,
After a period at rest you may switch over to the starting battery or 'ALL' before starting. However, if you go from the starting battery to 'ALL' after the boat is running, its very important that you don't pass through 'OFF' once the boat is running. Its potentially damaging to your alternator if you are using a basic switch.
If considering an ACR/VSR (automatic device) do a little research before your selection. Our favorite set-up is a SurePower voltage sensing solenoid used in combination with a Blue Sea dual circuit battery switch. We also use a different wiring schematic. This package better addresses every scenerio and closes any loopholes found in other products.

David
Earmark Marine