Exhaling into the bag has the theoretical advantage of keeping you neutrally bouyant, the problem is that much of the gas misses the bag and it takes a few breathes to launch it if you are shallow and/or have a large SMB. So when you inhale again you have what is in the bag plus what you just inhaled and you are then positively bouyant.
My preference is to dump gas from the wing into the bag via the inflator. The transfer is almost 100% efficient so when the wing is empty you are still neutral. You can then hit the inflator with the dump button still depressed and send air straight into the bag. And when you overcome inertia and just start to rise, release the bag as well as the dump button, but keep the inflator depressed. Upon bag release you are suddenly negative which kills the rise, and in the few seconds it takes to overcome inertia and start to sink, you will have added the gas back into the wing and will again be neutral. The result is very little change in depth, even whan launching a large lift bag or large SMB.
The thing that takes practice if holding the bag by pinching the bottom edge of it between the index and middle fingers of your left hand while holding and operating the inflator with the remaining fingers and thumb of the left hand - and it does not work well with some of the generic inflators out there with buttons on the side and end of the inflator.
The reason for managing the bag and inflator one handed is to ensure the spool or reel and the line is held out in front of you with the other hand, clear of any parts of you that could become fouled. You do not want to go up with the bag.