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All files are not flat tho. Check it against a known flat surface.

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Pat

I don't like files for leveling either because they are not level.... To me level is something that's been leveled and checked on a calibrated surface plate. When discussing tolerances for our purposes it's important to also indicate over what span. We seek a level set of less than .0005" of error over a distance slightly greater than the first though the 12th on a bass guitar.

But.... for builders you are working with a new neck and brand new frets. If you level and shape your boards correctly and with precision once the frets are installed you should only have to very slightly kiss the fret crowns with 220 grit on a leveling beam again checked on a calibrated surface plate. Recrowning may not be necessary at all and sometimes isn't for us.

I only use files and the files I use have been selected to be flat but they never really are... for hogging off some of the fall-away from the 12th to the last. After the heavy lifting was done with a small, flat file with wooden handle glued to it I follow-up with short leveling beams that have been flattened on the calibrated surface plate to remove errors produced by the unlevel files.

Probably a good idea to make distinctions between repair work and new builds. For your new builds if you shape the board with precision nothing more aggressive than 220 paper on a leveling beam will ever be needed to level the frets. This also avoids putting scratches on the frets from files or more aggressive papers.

In my repair world we have to fix all manner of defects such as Fender style necks that ski ramp at the body joint.... or body humps from manufacturers and builders who don't flatten the upper bought or have geometry problems. Files can come in handy here when it's a very long, hard slog.....