Typical guitar fingerboard is rarely flat. Human fingers are certainly not exactly straight and it's more convenient to play chords on a bit rounded fingerboard than on a completely flat one. There's however one good reason for making flatter fingerboards and that reason is the possibility to have a very low string action and to be able to bend all the strings without fretting out the notes because when you're bending on a round fingerboard you're climbing the string up, but it still remains in the same height in the bridge.
Some guitars have compound radius fretboards. In this case the freboard is round in first positions for great chord playing and flat in upper area for easy soloing and bending.
It's possible to flatten guitar fingerboard. Small radius change is possible without refretting the instrument. The difference in height is so small that you won't notice that youy fingerboard is still a 12", but the frets leveled to 16". What you will notice is that with string action as low as 1 milimeter at 14th fret for high E string you can bend wherever you want without loosing notes. That's what I did with B.C. Rich BICH that I own and it was worth it.
Fingerboard radius sometimes is not known, you can use such gauges as these below to check your fingerboard radius. Print, laminate and make sure that 1" line is exactly 1" long. Then try them out - one should fit your fingerboard.
Numerous guitars are equipped with bridges that allow individual per string height adjustment. That is not the case with popular Floyd Rose and Tune-o-matic bridges.
Use understring radius gauges to adjust or adjust bridge radius. You can also use them to learn fingerboard radius.
Typical Floyd Rose and Tune-o-matic bridges have 12" radius which isn't perfect for flatter fingerboards - there's no good reason for both E, A and B strings to have lower action than middle D and G strings.
Tune-o-matic bridges can be pretty easily adjusted to the right radius by deepening saddle slots with a file.
There's a really cheap alternative to professional nut files which, as you may already know, are expensive. Browse e-bay for "nut files". You may find sets of files designed for cleaning gas jets which are also great for cutting guitar nuts. Complete set costs only 2 pounds.