This is a misconception that people persist in thinking. You, as a good cat parent, need to be able to touch and feel all of your kitties body parts. Their chests, bellies, toes, etc, your fur baby may be reluctant at first but this is important to discover lumps and abnormalities As well as bringing you closer to your cat.

The easiest way to get your cat ok with petting her/his belly is to stroke their chin and then their chest. Soon and sometimes even on the same day you can reach down to their bellies. One of the main things to remember is if you touch their bellies and they grab you with their claws or their teeth – DO NOT PULL AWAY. If you pull away, even slightly they will think you want to play and they will dig their claws in. If you freeze and say “No Play” or “No Claws” then they will usually stop. (Or will learn to stop). The less you pull away, the less they will try to play.
These principles also apply to touching your cats toes or claws in case they need clipping and/or attention.
Remember that in order to have a good relationship with your kitty then you need to be totally in tune with her or him and know everything that might be wrong. Being able to touch them everywhere will help in keeping them healthy..
p.s. Patzy loves her belly rubs and shows me her belly all the time. Of course, chest rubs come along with belly rubs too and she loves those. I don’t focus on her belly too much or I might over-stimulate her leading to my reminder of “No Claws Allowed”.
This also makes vet visits less stressful for them as they are accustomed to people touching and feeling all over.