It surprises me that nobody has so far mentioned correct weighting.
If you are correctly weighted then the bubble in the drysuit at the start of the dive should not be that big. My steel 15L tank changes its buoyancy by less than 3kgs over a dive. This should equate to a bubble with an "at depth" volume of less than 3L at the start of the dive, and it gradually gets smaller over the dive. But, correctly weighted, this should be the same volume as you would put in a BCD. (actually at 200bar my 15L should lose 3.6 kgs full to empty, but it is rarely either completely full or completely empty)
Each diver for his equipment *should* over time start to know how changing undergarments / tank / other equipment changes the size of the bubble, and how they weight themselves.
Now, how you do your buoyancy it depends on the bubble size. Single tank diving (upto and including an 18L tank) can be easily controlled on just the suit, as can small twin tank setups (twin 8Ls). However, when you start having larger twin tanks such as twin 10s or 12s, then the bubble volume gets larger and harder to deal with, simply because you have a greater buoyancy change as you use up the air.
Personally, as I descend when I know I need a large bubble (with a twinset for example) I will take off the squeeze with the suit, and add air to the BCD. As the dive progresses I will do the majority of the boyancy on the suit, but gradually reducing the air in the stab.
OTOH with a single tank then I just do everything on the suit.
Another minor point is that all divers are different, and I for one find that getting air into the legs of my suit quite difficult. Perfectly horizontal I have the bubble around my back, and it is actually quite hard to get the bubble to go past my weightbelt. This leaves me quite nicely trimmed. Also, with a thinsulate undersuit on, the bubble is less mobile as the air appears to be "stuck" in the undersuit, although it does not affect purging or inflating the suit. (hard to explain, but the undersuit compresses to quite thin, but when you put air in it springs back to its normal thinkness, and the bubble doesn't move around as much)
As said many times above, stop, think, optimise, and do what works for you.
HTH
Jon