NOTHING reveals contour like reflection. Undulations down to a few millionths of an inch are visible to the unaided human eye (presuming good vision to start with) under the right conditions. The opposite is the case for matte (non-reflective) surfaces... fine gradations of depth are almost impossible to see. In fact, near-white, sanded wood is almost a textbook example of a material with "shape camouflage" built into it.
The way to make reflectivity work for you when shaping is as your final shape is approached, take the time to put a gleam on the key areas with a bit of compound (for stems), or a bit of wax (for bowls), and examine things under a bright, single point light source. Tilt and move the work slowly, looking only at the reflections. Then mark the HIGH spots---remember, pipe making is 100% subtractive---with a soft lead pencil or other suitable tool, and sand some more. Check again when you think you're there. Rinse. Repeat. (Depending on the contour, sometimes it works best to mark the low spots so you know where not to remove material.)
A defect-free surface is not necessary when creating "check shines," by the way. You only need to get some light bouncing off the surface.
The best way to keep track of your shaping before reaching the reflection examination stage is using a low angle shadow. Use the same bright, single-point light source, but instead of looking at the gleam lines, hold the work in a way that casts the longest possible shadow over the surface you want to examine. It's the same way drywall installers check their work, and termite inspectors check for damage, by holding a flashlight almost parallel to the wall. (Yes, It is possible to see termite trails UNDER wallboard using this method, it is astonishingly sensitive & effective)
Happy sawdust-making.
I'll have to try this sometime; thanks for the tip!