Rarely IFR, it only takes a moment to spin the wheel and check time/distance/speed stuff...not like you're head down for any length of time.
Well I guess you can do that. I usually approximate those things to +/-2min (or +/-5min for greater distances) given my groundspeed it correct.
This, I use it quite often in VFR for computing groundspeed
How? To do that you need your winds, if your aircraft can tell you the winds than it can also tell you your groundspeed, if you don't have your winds than you need to use the winds that you checked before T/O therefore you might as well get your GS before T/O. The winds you ask the tower are on the deck, you can also ask FSS but they can only tell you the forecast which you could have got before T/O yourself.