BKL mounts are very good if you swap a scope between rifles i.e. the centering is pretty much spot on as long as you take care when tightening the mounts to the dovetail (there are others who do the same centering idea). So great if you clamp the rifle and want to try different scopes and do a quick pre-zero alignment before firing a shot and decide to put the original scope back hahaha. They come in limited heights though so you may need risers.
Scope rails typically come with a small but essential slope which helps keep the scope optically centered i.e. you keep the turrets near the middle of the range. You may still need an elevation adjustable mount (e.g. Sportsmatch) but this depends on where you want your "zero" as Simon said. There are also elevation adjustable rails.
BKLs have always been great in my experience but equally I have had Sportsmatch mounts give me an offset (i.e. switch a pair front to back or mount one bolts to the left or right and have the scope show big "windage" changes). Experiment and eventually you will have a combination which is stable and of course - no zero shift after use or travel - don't over-tighten bolts but you don't want them to loosen either :-)
You can spend hours (days) fiddling to your heart's content with this e.g. slacken something just enough so you can slide the scope/mount/rail back and forward - it should remain on the same vertical line and rise and fall if you have a slope assuming the rifle is level, mounts level, not too much loose, etc. If you rotate a scope inside the mounts some have the image rotate exactly around the cross-hair, some have a rotation some place else - I never worry about that one as it makes my brain hurt pondering what it might mean :-)