VNAV is useful but you should also anticipate shortcuts. VNAV will assume that you are going to follow the full route loaded into the FMS, including any RNAV transition, and calculate your descent path accordingly. If ATC then gives you a direct track to intercept the final course or something you may find yourself above profile.
Using the 3:1 rule (around 3000ft descent over 10NM on a 3 degree glide) you can mentally calculate a rough path. If it's not busy expect shortcuts. After flying to an airport a few times you get used to what the ATC is probably going to tell you and can plan accordingly.