I have a 40' long I-beam in my house with a t&g ceiling. I'm traveling right now and can't share a photo. Seems like you'd want to cover the bottom of that beam, no? Or does the beam get flipped over?

It gets flipped over. That top in the pic is actually the bottom you would see.

That bottom really makes it look fake, which it is. Even if it's not mitered, it could be flush which would look better, but if you can miter it, then do so. If they're being painted anyway, then you can spackle them before painting to hide imperfections.

I agree with Annie, that bottom should be flush.

Yep, I agree with Annie. And sadly, the original "beams" in my 1970s house look like this and I want to replace them with mitered stained faux beams. If I wanted them painted, flush would be just fine.

If it's the recessed bottom that you aren't sure you like that's one thing, but if it's the mitering, I don't think you can tell the difference when they are up and painted. My dining room ceiling is a 1918 arts and crafts style with that recessed bottom beam (stained & original) and there is no way I could tell if they are mitered without taking it apart. Assuming what you are talking about is the side boards being flush, rather than having a groove for the bottom board to fit into. Also if you go to Houzz, there are stylish homes with the recessed bottom beams.

The miter would make that bottom board flush, vs recessed. I'm working with a bungalow arts and crafts style home circa 1950's. Those beams were typical in that style as well. Is there any chance you could share a photo liz? I've got 3 beams built and ready to be primed and painted. Did a small groove at the bottom so it is recessed less than the photo shown here. They didn't turn out as bad as I was thinking and I may be sticking them up. Going to have the husband hold one up so I can get a feel for how they will look. You are right, after browsing Houzz and seeing that style beam in many of the beach bungalows and arts and crafts homes, I felt much better about it.

The recessed field of the soffit goes back to antiquity; the greeks cut their marble lintels into a fielded/recessed shape and decorated the inner edge with carved moldings.
This was picked up on by woodworkers creating classical-revival interiors an exteriors. This would typically be a spool and bead, or "sausage & bead" type of running carved molding, or could be as simple as a transitional cove shape, or flat ogee.
Don't diss it as fake when an important element, the molding, has been stripped away, thanks to tract builders who could not care less.
Casey

I think if you want it to be as authentic as possible go with flush. Recessed were used in Arts and Crats style occasionally, but these are hollowed out faux, so making it look as genuine as possible is key. The recesss is overkill. My vote is flush.