Dr. Who is one of my favorite pins, and probably one I've put the most time into learning. A lot of people start with Doc #6 (Colin Baker), which increases scoring in a general way across the playfield. If you're going for multiball, you'll want to start with Doc 7, which will help you out from having to bash the unsafe Time Expander as much.
My strategy whih has helped me score over a billion consistently is to start with the Doc that increases the multiplier at the left lane (the one that feeds the upper-left flipper) - that's Doc 2 or 3 IIRC. At the beginning of the game, the ball feeds from the TARDIS to the right flipper; I shoot left lane, upper ramp loop 10x which feeds to bottom left flipper, and shoot the right ramp for the TARDIS Combo, and repeat. At some point when your multiplier is maxed, use the opportunity to shoot the multiball locks (you can keep the combo going to get the ball to either flipper and make the lock you need). Then, I start shooting the upper inlane that feeds the pop bumpers and TARDIS, to light video mode (which will award extra doctors). Doc 1 will help you get to video mode faster, so you might want to choose him if you need score but are having trouble making the ramp shots or can't keep control of the ball while bashing Time Expander.
The main goal at all times, for me, is to have the multiplier maxed before starting multiball or video mode - that's what moves my score up from 200 million to 1 billion +. If your shots are inaccurate, you'll probably light some extra video mode targets to the left of the TARDIS, or you'll bash some Time Expander targets on your way to multiball. The main issue is losing control of the ball, as the outlanes can be pretty rough on this game.
Hope that helps!
-Stephen