Busflyer,
Whilst in HDG SEL, if your cross track is only 3 or 4 NM, you’ll find you’ll meet your waypoint altitude constraint using VNAV. Much more than that, and start keeping a close eye on it. To do so, enter the name of the waypoint on the “FIX” page. Then select the “ABEAM” prompt. This will shoot a dashed green line perpendicular to the aircraft track. Then you only have to ensure that the green “Predictor Arc” lays over or in front of the “ABEAM” prompt dashed green line.
~Taco