Is there a preferred mapping of Game Boy buttons to keyboard keys?
The preferred mapping would be to allow the user to decide, but here's a few common options.
WASD Layout
Arrows Layout (ZX)
The two above run into issues with some keyboards having different W and Z keys, if your engine uses character inputs instead of keyboard positions. The following avoid this problem.
Arrows Layout (XC)
ESDF Layout - This is my favorite because I can use the ridges on F and J to find the right hand position, and for me it simulates the "feel" of a gamepad. If I'm stuck using a keyboard for an no-mouse action game and can modify controls, they usually look like this.
I'd definitely like to hear about other people's preferences, though. The only way to make every player happy, of course, is to let them decide their own mapping.
Happy jamming!