When I have something (let's say 50 px long) that I want to make longer (90 px), inkscape doesn't go immediately to 90px if I enter it in the Width field. Instead, it goes to 80.7, then to 89.6, then to 89.993, etc. It takes several entries to make it 90. How can I eliminate this behavior?

I've read posts by others having the same problem but never experienced it--and I can't remember what the concensus was in the post. It sounds like a bug. You might like to check if has been reported on Launchpad. You could also try the development version of Inkscape and see if it has the same problem.

I think using Geometric bounding box instead of visual can help there, it's in Inkscape preferences. Geometric box means you'll have to calculate for stroke width yourself (if you have stroke on the object in question).

It's not in the devel version, so I guess it's fixed.

Yeah I've had it a few times. I can't replicate it of-course. I think it happened more with path which had stroke and fill and that I've scaled up and down a lot by hand.