It's slightly easier to do it with the neck off the guitar. Also, if you want to 'roll' the fingerboard edges, then it's best to do it before any finish is applied, as you will certainly remove some of that (although if you want a 'reliced' look then after the finish is on will give a more heavily played look). But it's almost as easy to to with the neck on - though you'll need to support the neck from underneath when sanding/filing. With it off the body, you can just rest it on a towel or a cloth etc. on a table or bench.