Howdy.
In the past few years, I have really settled into focusing on playing steady time. When I practice, I use both metronome, drum machine as well as prerecorded loops or samples. In addition to those tools, I also always practice along to some CDs.
One of the worst feelings in the world (for me) was the realization that my time wasn't just naturally "perfect" after all these years. I had just ASSumed that I had that sense. Mind you...my time wasn't that bad...but it was far from "perfect".
A machine, like a drum machine or a metronome is designed to meter out a pulse that is unwavering. By using these as tools for practicing, one soon learns to trust in that machine-perfection. It's a very reassuring feeling to have a "partner" (the machine) that doesn't ever screw up! ;) It makes it easier to do all those cool metric modulations and meter-over-meter patterns without ever pushing or pulling the time out of whack in the process.
But...
...it also makes it frustrating when the time comes to actually play a real, live gig with real, live "wavering" musicians. Suddenly, that "crutch" is gone and YOU are the one who gets looked at strangely when the heads come back around at a different tempo.
Practicing with a machine reinforces the feeling to evenly meter time that all we drummers are compelled by -it's essentially the basic nature of drumming.
However, real human beings tend to "place" the time either in front of the actual "perfect" time (of a machine) or behind it. If, as a drummer, you are used to hearing that machine metering out perfect time and then you go to a gig where the bass player is dragging and the saxophonist is phrasing with the bassist...the piano player is rushing...etc., then the time starts to feel "funny" -like you're pushing/pulling on them...when, in fact, you may not be pushing/pulling on them. But that is the perceived feel that comes across as an overall result of that particular combination of players...and it's why some bands sound so great even when the individual components of that band are not so "perfect". Sometimes, it's the combined inconsistencies that make the groove feel so good. It's something that can work for or against the sound of a band.
It's not a sin or even that big of a deal, really, if the time changes. If the music is happening, then it's one thing. And, since different people hear things from different perspectives, the evenness of the time feel may not be an issue to everyone. I remember one old guy telling me, "I don't care if you rush, but, man, DON'T DRAG!" when I was about 19 years old and playing in a dive-supper club....and wearing a tuxedo....but, I digress...
But, for me, I will continue to pursue "perfect" time, anyway. It's what I like. Even if none of my gigs afford me the opportunity to play "perfect" time, I will still be thinking of how to maintain it as evenly as possible -whatever the odds against me might be!
One new "excercise" that I like to use in my practice, is to start the metronome at 40 BPM and then play the pulses with alternating strokes (L-R-L-R) about 8 or 16 times...
...Next, sub-divide that pulse into two strokes per pulse...Then, play three...Then, four...five...six...seven...eight...nine...ten...eleven...twelve...thirteen...(I have only gotten to thirteen so far!)
Try to play the strokes as evenly as possible. Do them starting with the right hand lead and then do them again starting with the left hand lead.
Then I move the metronome up to 50 BPMs and repeat...do as many subdivisions as possible at each tempo.
:)