I use the maxim of no wider than 6″ max. I have never had a bottom crack, warp or distort using this as a guide. It’s also a good idea to acclimatize the wood by allowing free air to circulate around all faces of the panel to be glued, whether inside a groove or glued. A day in the environment it will live in is good.My North Wales workshop is absolutely climate controlled with humidity leveled at 48%. The temperatures are 70-degrees. I keep wood in stock generally, but sometimes I must buy in, even from unknown sources. In this case I use both moisture meters and weight measurements to know what levels my wood is at before I start a project. Board footage to weight per cubic foot/metre works well but you must know your woods to know what weights you are looking at. I also encourage experimentation and so dry out different woods to zero to see what size they reduce to and then allow atmospheric moisture to reenter and see what expansions settle at. That way, you can assess what you want for your home. Simpler still is to take a board into the home, office or workplace and leave it there until it remains unchanged for a week.
Looking inside antique tool boxes, jewellery boxes, and a range of other such pieces, it was common to these projects too. Over 6″; always use a groove and let the panel float constrained.