Is it normal to get slight sag when pulling the center prop on decorative trusses?

You shold definitly plan on at least 1/16" to 1/8" of sag once you pull that center prop if the assembly is only supported out at the wall plates. Even on purely decorative pieces, the dead weight of heavy timber will cause a tiny bit of **joint slip** around the bracket bolts and dowels as the steel bites into the wood. The timbers themselves will also exhibit minor deflection over that span once gravity takes over.

The standard way to keep it tight against the ceiling boards is tying the top of the king post directly into the roof framing above. Don't rely solely on the wall connections to hold the center flat against the ceiling. If you have attic access above that tongue and groove, run a couple of long structural screws or all-thread down into the apex of the truss. If the ceiling is allready closed up tight, drive countersunk structural screws straight up thru the king post into your ridge or ceiling joists, then plug the holes with matching end-grain wood plugs.

Do that while your 2x4 prop is still wedged in place. Once the top is tied into the ceiling framing, ease the prop out slowly with a hammer instead of just kicking it loose so you can watch how the joints settle. A tiny hairline reveal against the tongue and groove isn't the end of the world anyway—seasonal humidity will move those ceiling boards regardless—but mechanical fasteners at the apex will keep it from drooping.

Put a slight preload on it before you drive your final wall fasteners. Whenever I hang heavy decorative spans like that, I use a bottle jack under the center post to bump the crown roughly 1/8" to 3/16" past flush against the ceiling. Once it's pre-loaded, drive your 5/16" x 6" Simpson SDWS Timber screws into the wall blocking.

When you ease that jack down, the bracket bite and timber deflection pull it right back to dead zero against your tongue and groove. If you just screw it off dead level, leverage works against you—1/32" of bolt slop at the wall pocket easily translates into an 1/8" sag at mid-span.

What species are the timbers and what was your moisture meter reading prior to hoisting? Everyone focuses strictly on mechanical deflection, but if those chords are unseasoned or even partially green Douglas fir sitting at anything north of 15% MC, the cross-grain shrinkage you'll experience through the first winter heating cycle will easily dwarf an initial 1/16" joint settle. Tangential dimensional loss across an 8x8 member will open up a noticeable reveal against the ceiling boards regardless of how tight you pin it today, so you might drive yourself crazy chasing dead-level tolerances against seasonal wood movement.

When I did mine, it dropped maybe 3/32" no matter how tight I bolted the brackets. The trick that saved my sanity was stapling a strip of black roofing felt along the top face of the chord before lifting. Even if you get that tiny settle against the tongue and groove, the dark strip turns any micro-gap into a clean shadow line instead of an ugly light gap where you see unvarnished wood. Made it look completley intentional and saved me from losing sleep over an eighth-inch drop.

Quick update — went with the bottle jack preload trick this morning. We bumped the center up about 3/16" tight into the ceiling, sank the rest of the structural screws into the blocking, and eased the jack out. It settled down maybe a heavy 1/16", right where it needed to be, completley flush against the tongue and groove. Relieved is an understatement. Thanks for the help!