There are special considerations in fretting a neck that is or is to be finished including the fingerboard surface. Maple is the most common neck and fingerboard material that is finished as it lacks the density and oil content of rosewood and ebony etc and is thus finished to protect it from climatic change and decay.
There are some builders and players that prefer an unfinished maple neck and fingerboard; such an arrangement leaves the maple prone to more expansion and contraction and thus more maintenance over time. For those reasons I do not recommend an unfinished or tung oil or oil/wax finish on a maple neck as it doesn’t protect the substrate well enough (body is another matter and less of a structural or playability concern). A stable neck is one that doesn’t deviate much from where set and the purpose of finish is to protect the neck (already under the pull of gravity, strings and truss rod) from climatic influence.
Should a maple neck be finished before or after installing frets? If a maple fingerboard needs to be leveled or it is desired to change the radius, then the fingerboard must be refinished.I do it both ways depending on the situation.
There is an additional cost for finishing maple fingerboards during refrets as it is more time and work to complete.
In the end for me it is all about what the client wants for his piece.
Below are some pics of a mid 1970s maple board Fender Strat refret in process; the board has been defretted, reradiused, prepped, refinished and new frets installed over the finish - these pics are prior to the final fret leveling and fret end shaping (but they will be left fairly straight as the OEM bridge has the wide 2 7/32" string spacing for the stock 2 3/16" heel width neck, so the strings are close to the edge as designed/built by Fender - common until the arrival of the American Standard series in the 1980s). Only the fingerboard surface has been refinished - the rest of the neck finish is original.
A note about 1970s Fenders - the finish for the headstock face is nitrocellulose lacquer due to the fact that the C transfer decal does not do well under Fender's Thick Skin Polyester finish that is used on the rest of the neck (prior to ~1968, Fender used lacquer for all finish and the decals were applied on top of, instead of under/in the finish coats as in the post 1968 period), so the darker, more amber color and different thickness/depth, tint and sheen of the headstock face, as well as the bleed over the headstock's edges, are readily apparent. We refinish the FB surface to match the rest of the neck as Fender originally did. The contrast between the two finishes varies widely among Fender's output in this decade. I took these shots in the best light possible to show the neck and the FB surface refin as best as possible. Note too that this is Jescar's FW55090s, a Stainless Steel fretwire, and as such, I have opted to recess the tangs due to the difficulty in smoothing the tangs at the neck edge w/o finish damage. My opinion is that the recessed & backfilled SS tangs feel better than if not done; this is generally not necessary for Nickel Silver fretwire.