A strapless dress needs to be skin tight at the waist. The waist is the narrowest part of the body and making the waist fit perfect will ensure the dress cannot slide up or slide down. It will stay in place at the smallest part.
If it seems tight, but you’re not sure, grab the waist of of the dress and see if you can pinch excess fabric. We usually do this test at center back.
Another test for the waist is to simply put your arms up in the air and jump. If it slides down, it is too big.
Straps alleviate the need for tight bodices. However, making the bodice tight will make you appear skinnier.