I’ve always thought that, in general, Time Lords/Gallifreyans that have the ability to regenerate usually have great control over the process, it’s just that 99% of the time all the regenerations we’ve seen have been as the result of fatal injury, y’know; death. The one instance of on-screen regeneration that was done completely by choice, not at all as the result of a fatal attack, was Romana, and she could choose/morph between a few bodies before she settled on one.
So in other words, the Doctor would be able to choose exactly what he would look like post-regeneration if he did it at normal/full health, but he only ever does it when he’s about to die so he just ’goes with it’ and gets randomly assigned features.
The only thing that might contradict that is the Masters “Derek Jacobi to John Simm” regeneration, but I reckon that it’s implied in that episode that the shot the Master suffered wasn’t immediately fatal, it’s just that it would be easier or more convenient to regenerate than to risk stopping and finding medical attention (he was a renegade TIme Lord, with an aged body, running from the Doctor due to being discovered; too risky to expose himself, I assume). You see him ‘choose’ to regenerate, mentioning “if the Doctor can be young then so can I”.
[Note this is only based on ‘canon’ DW, not expanded universe novels, comics & audio, which everybody knows (or should know) don’t officially count and are constantly supersceded by the show. Not to mention the number of times a 50 year old show can contradict itself, a fluid continuity indeed :P]
