I try to keep them level all the way across by watching the width of the flat I am filing across the frets. Of course - I also measure from the top of the fret to the fingerboard as a check. This assumes the fretboard is perfectly surfaced.

I think files are way too aggressive and crude for this work. I use a 24" carpenter's level that has smooth sides with sandpaper glued to them. Start off with 220 grit then move up to 320. Just as good as a high price leveling beam. I've been using this level for over 35 years and it has done hundreds of guitars.

What works for you use as a levelling beam? the guy that taught me uses a smooth bastard file epoxied to a square piece of wood. It seems to work pretty good, but it does leave a lot of chatter which I then clean up with progressive grits of sand paper.

Conor - I use a specially made fret leveling file given to me by Dan Erlewine, a sandpaper file with a glass sole made by Ken Donnell, a diamond leveling plate, an ancient Nicholson smooth bastard file that I safed back in the late 70's, and a very hard oak sanding board wrapped with sandpaper.

If your file is leaving chatter marks - it's possible your technique that needs to changed.
Are you using too much pressure?
Are you aware a file only cuts in one direction?
Do you lift the file as you move back for the next cut?
Is the file brand new? New files can actually be too sharp.
Are you left handed? Most files are made for rightys...

Let me know...

All of those every time!?! Or do different jobs dictate different tool choices?

I don't think anyone would notice if there is a discrepancy in fret height (unless too low of course) or radius.

I think he means discrepancy in fret height across the fret i.e. the frets being slightly taller on the bass side but all the tops being level with regard to the string paths.

That is quite a bit wider than you need, but it should still work.