Ultimately it’s the modelers choice of course.
I paint all the exterior surfaces, back side of road wheels, under the side skirts or bottom of the tank, all of it. For me it’s 2nd rate effort unless I paint all of it.
Starting on the bottom of the tank model is a great “warm up” if one hasn’t painted in a few weeks. I can make sure all is in order with paint mix, air pressure, airbrush etc before working on any area exposed to view. It’s the ideal area to experiment with shade and fade etc. Using 15 cents worth of paint on the bottom has probably saved models from a compromised paint job.
Wasting paint on bottom of tank argument, is at best a trivial concern when painting a $50 to $80 model with 40 to 120 hours of build time. Worry over maybe 15 cents worth of paint? Not me.
Mix 1 or 2cc’s of paint for the airbrush at a time typically. Basically little no waste painting the bottom. It has very little impact on my paint usage. With a good gravity feed airbrush one can mix drop by drop if needed.
I still use Floquil RailRoad Weathered Black that costs $50+ for 1oz bottle on Ebay these days, I’m definitely on point with conservation of precious vintage paints and still paint the bottom of the model.
Despite being basically “retired” from building for contests or judges, I still strive to maintain building and painting to IPMS contest model standard. Exposed unpainted plastic is on the same level as failing to remove exterior punch marks on parts to my point of view.
I know that isn’t for everyone but that’s my thought process.