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Depending on the many variables applied when pulling a shot of espresso, the rate and consistency of the flow of the espresso varies.
Does variable speed or consistency suggest that a shot will be of lower quality? Conversely, does a steady speed and flow indicate that a shot will be of higher quality?
A more watery/lighter consistency with a high flow rate is going to leave you with a terrible tasting shot of espresso with too light of crema (this foam on top can tell you a lot about a pulled shot even if it does look as if it came out OK). The opposite applies here to, a super dark and slow pouring shot can occur too.
In my 4 years of coffee house experience using professional grade equipment it all came down to the grind size. You can tell just by the way the shot pours out. If it's quick make the grind a little finer, if it's slow make it a bit more coarse.
And also depending on the temperature in the room can effect the pulls as well, half way through my morning shifts I would have to adjust the grind.
I'd say yes...
I would not categorically claim that a steady flow will give you a better shot, rather I'd say it gives you a more consistent shot; and consistency is highly desirable—if you can reproduce a shot consistently, then you can incrementally improve on it by tweaking one variable at a time.
Water pressure is a key variable in extracting the oils from the coffee, so if that variable is moving around during the extraction of a shot then you're going to get an inconsistent shot—it would be like mixing weak and strong shots and stirring them together.
If the flow rate is fluctuating then—I can't think what else it could be—unstable water pressure is the cause. Water pressure fluctuations could be the result of:
A variable rate of flow could get interesting if you have it under control and are experimenting with extraction techniques. You might try to slow it down toward the end of the shot in order to get more oils out, but I think this just makes it bitter because you're actually reducing the pressure—so it is self-defeating. Unless you have engineered a device which can detach pressure from flow rate... but now I'm dreaming... something like a camera shutter mechanism?
