Yes. I typically do it on descents to keep up with the diminishing volume of the wing from added depth.
It's a skill and like any other skill, I like to periodically practice it. If on OC, it helps get a feel for how much air you can exhale and still sufficiently clear a reg and it's also helps keep me comfortable with reg switching. Spending time with a reg out of my mouth in a controlled situation reduces the stress of this action if I ever find myself in an uncontrolled one.
On CCR, I don't do it as much, but I still do it some. It helps build muscle memory for closing the loop when switching to bailout.
Since I hijacked the thread a bit, let me answer the OP's question. This came up in another thread last month, so I"m just cutting and pasting my reply to that below.
Guy
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Reading this a bit late, but for some value of "experienced" (270 dives), I'll take a shot since no one else has. Personally, I think that you'll break even at best on your air consumption; every time I do this underwater I always lose a little air that's trapped in my 2nd stage when I pull it out of my mouth, and if I haven't remembered to retain sufficient breath to clear it when I re-insert it, I've got to use the purge and waste more gas.
But I still think it's a good idea to do this once in a while (or better yet once a dive), and I always inflate my reg on the surface manually (I test both manual and power inflation before the dive). Manually inflating it saves me a bit of tank gas which might come in handy If for any reason I need to re-submerge; it's kind of like the old aviation adage about there being nothing more useless than runway behind you.
As to using the manual inflation underwater, I try to do it to keep the fact that I _can_ do so in my mind. I had a solo dive some time ago where I'd been a bit hurried in my pre-dive prep and had neglected to hook up my inflator hose. The dive was over a hard bottom and the depth was modest so no big deal, and I just hooked it up on the bottom when I realized the problem.
But, while I was descending, my brain wasn't really thinking about _why_ using the power inflator wasn't slowing my descent rate (I was descending horizontally, so kicking wasn't an issue to slow me), nor was it processing the lack of noise. And it never occurred to me to use manual inflation until I reached the bottom, when I no longer needed it. Since I feel that good habits are best ingrained by constant repetition, using the manual inflator underwater on a fairly regular basis seems to me to be a good idea, even if I use a bit more gas in the process. Even though I normally dive dry now so have redundancy, should I ever have an inflation hose issue again hopefully my first response will be to try to manually inflate, instead of puzzling about what the problem with the hose is.
And the other, and even more important good habit I re-learned from that episode is that whenever you're hurrying through your pre-dive prep, Stop! Take a deep breath, slow down, and be extra meticulous about doing the inspection right.