Typically without question I belt any tunic. But through some experimentation today, I discovered this is not always the case, as shown below. I've come to the conclusion (for my body type), it's best to leave a fitted tunic un-belted to avoid a muffin top.
On the days I feel like sporting my trusty muffin top, I will definitely reach for a belt.
Boots: Steven Lovve via Ideeli
Jeans: Joes via Neiman Marcus outlet
Steven boots (via Ideeli)
Joe's Skinny Visionaire (via Neiman Marcus outlet)