I pick random exits and entrances on each room and just try to drill from exit to entrance.
I tried drilling from exit to entrance straight and generally that looked awful, most of the rock got carved right out in the center. Then I remembered about the way you can make certain paths "expensive" to a search algorithm.
When the driller goes to select a new square a penalty is added to the distance of any previously visted square and to any untouched squares. Thus it tends to double back trying to use less energy, gets trapped, then digs out - or runs out of time.
I think the parallel routes and favouring of corners work well for my game with it being a platformer.
I'm now trying to figure out how to cover it all with ledges and ladders.