Engaging a muscle a lot of the time is not a good thing.
Constant holding creates strain, weakness, tightness, mental stress too.
Your skeleton will hold you up. You won't fall apart if you relax.
So what took so long to realize this!? I always thought I should hold my belly in; loads of pressure to have a flat tummy.
But I've been experimenting lately (a little embarrassing with it being summer, the season of little clothes) and have found that as the belly is going, a new sense of calm is coming.
I never knew how much anxiety could be held in the belly, but there it is.
Give it a shot. If you are worried, wear a flowy and loose (and fashionable!) dress or shirt, and completely release. Soften your belly. Relax your belly. Let energy and circulation move through your belly.
Can you imagine the Buddha with a flat belly? No way!