I think it's a great skill when a drummer can lock in and hold it there until the end. A few BPM variation without a click is about as good as can be expected.
In my trio band, the leader...his time is as good as it gets. He is the only guitar player I know that practices to a metronome...Sometimes he gets too Nazi with it though. An example: When the bass player takes a solo, playing with the meter, ever so slightly, and only when the soloist themselves go there first, is awesome. This is where we disagree on time.
Not too long ago, the bassist was taking a solo, it was just her and I, no guitar, and she got to a very delicate passage in her solo, so I lowered my dynamic, and ever so slightly, held back the time just a tad, for effect at the very "trough" of her solo. She actually was in control of the solo feel, I was just mirroring her as best I could. She made her solo breathe, and I just went where she was headed. I liked the effect, it was very gentle and loving in a way. So I was feeling all warm and fuzzy. Well the leader started moving his guitar neck to "metronomic" time, like I was dragging. Now remember he was laying out at this point, it was just the bassist and I, and she didn't have any issues with my support. In that case, sometimes time sounds good when it breathes. As soon as she left that delicate place, naturally I upped my dynamic slightly and returned back to a non stretchy meter. I basically ignored the leaders "correction" attempt. He wasn't playing, so I did what I felt was best, and he didn't agree.
Ever hear an unaccompanied piano player play with the time? You know when they hold notes or chords for effect? To me, not only is that totally legitimate, it indicates a high level of time feel control. Sometimes to create an effect, the time has to stretch. When the full band is playing, in my world, stretching the time isn't practiced, god forbid, but if I'm backing a soloist, meaning it's just them and myself playing, then stretching time....ever so slightly.... can work well to create moods, but only if the soloist goes there first. I don't dictate the soloists time feel in that situation.
But back on topic, I never actually measured myself. What I do is on every song, when I get near the end, I mentally compare the ending tempo to the starting tempo, from what I remembered the starting feel to be. I work on my meter with a metronome enough to feel that I am steady enough for my purposes. There's no huge variations.
Metronome practice really smoothed me out and I think all musicians, not just drummers, should practice to a met until they understand exactly what steady meter sounds like.
