or almost always. Certainly when starting out. Developing a good sense of time is so important.

Everyone has a different philosophy about practicing, I'd say if you really want to make progress on an instrument you need to practice everyday for a minimum of 30 minutes, the more the better.

If something is too difficult, try slowing it way down, which can be difficult in it's own way. My metronome's slowest setting is 40 bpm. Playing 16th notes (or better yet 8th notes) that slow really shows you where your bad tendencies lie and help you to correct them.

To improve speed, find your upper limit for doing it solidly with the metronome, and set it up a notch or two and work it out.