This approach would not only look weird, but it would look downright bad IMO.

Are these openings arched? Or square? How extreme is the width difference? Is it so bad a bead of caulk would not be able to disguise 99% of it?

The easy approach is to leave it as is. Or hire a skilled carpenter that can account for the difference in width of the opening.

Thanks sambah006. They openings are square (well, rectangular). I don't really know how extreme the difference in width is. What would you suggest a skilled carpenter would do in this situation? Would they cut the jamb piece into a non-rectangular shape to match the existing dimensions? Or use a wood jamb piece sized to the widest dimension and somehow fill in the difference?

Jennifer,your contractor is either trying to save you some money or simply isn't skilled enough to make the openings look the way you want. If it's the first,let him know you are willing to spend extra. If it's the latter,part ways now,the worst is still to come down the line. This type thing is normal while remodeling old houses which have had dozens of ideas tried on them.

Ack, that is not reassuring. We are pretty much committed to using this contractor and like him in general. I think he would be open to discussion if we can come up with a suggestion of another way to do this. Would the right way to approach this be to cut a jamb piece to fit the opening as it is now shaped? Or build up the walls somewhat near the opening to make them more even? Or use the trim to mask the fact that the jamb width is not consistent? I'd appreciate any suggestions. Thanks!

Useing plaster to even out the wall thickness would be the most preferable. How far out to carry new plaster from opening depend's on how much difference is being made up. My secound choice would be sizeing jamb to thickest portion of wall and make up difference with filler between outside edge of caseings and wall.

Filler? you mean caulk? or would there be something better?

Since op hasn't given us the luxury of knowing how out of square the opening actually is,we are shooting it the dark sam. If gap isn't more than one quarter inch caulk should do a fine job dressing things up. Much more than 1/4th and plaster would be better. Large,inconsistant gaps can also be partly concealed with an extra scribed wood molding. Contrasting colors between wall and mold should be avoided unless one is clever and skilled enough to make color work in their favor. All you are doing is moveing the mis-fit to the least visiable area. Any way it's done still require's skilled hands if it's to turn out satisfactory.

I measured and the discrepancies in width vary from 0.25 to 0.5 inches. Some of them just get thinner or thicker as you go up, some are hourglass shaped. thanks everyone for the suggestions. I am supposed to talk to our contractor about it on Monday so this is all very helpful.

0.25 to 0.5 should be easy to fix with a little sanding and plaster.. You have me scratching my head trying to visualize how he planned to cut the jamb in hourglass shape then fit caseing to it I'll bet 90% of people wish this was the toughest decision they incountered while remodeling a hundred year old house.