I use the gallery wrap function and it works well. You are right, you have to have the image the correct size for the stretcher bars. I determine the stretcher bar sizes that I want to use. Then from LR I export to PS. If the image is not the same aspect ratio as the stretcher bars, I crop setting the correct aspect ratio in the options bar at the top. Then open Perfect Resize from PS. In the resize part of the program I resize so that it matches the stretcher bars and is at the correct dpi. Then run the gallery wrap function.
I go a little further to make life easier when stretching. In PS, after the image comes back from Perfect Resize, I add 1/4" to the canvas size all around. The image size stays the same but the canvas is enlarged. I select all and add a black stroke to give me trim lines. Then using Guide Guide (a free action) I add guide lines matching the wrap that I used. If I have a 2" wrap, I set the guide lines to come in 2 1/4" (allowing for the canvas resize). This gives me guide line which are exactly on the boundary between the image and wrap. You can do this by eye but this is precise.
Using the guides and snap, I draw black tick marks on the 1/4" outer portion of the canvas with the pencil tool. There will be eight tick marks.
After the canvas is printed, I trim to the canvas size. Then with scissors I nip into the 1/4" area. Turn the canvas over and with a long straight edge butted up to the nips in the canvas, draw a pencil line. This give me a rectangle on the back of the canvas which matches the image size on the front. Set the assembled stretcher bars so that it matches the rectangle, then stretch.
No light table so this is the way I ensure that the canvas is positioned correctly to the stretcher bars. Not a production method but for the few that I do, it works well.