By rule of thumb, I use 1/16 inch, all the way around, depending on what I'm using as a spacer.
The reasoning for the extra room is that cross-grain (like across a wood panel glueup has) expands faster, more and at a greater rate than long-rain (as with rails)... so if you get a big change in temperature or humidity, you pop your joint apart between the rails and stiles.
The reasoning for using something that compresses between the panels and the door's R&S is that if it does have extra space there for that expansion ... and if you don't have something there, then the panels rattle when closed.
The old-school of doing that without, or rather before people started using some kind of compression filler, was to give 1/16" additional for each side of the panel and none extra for the top and bottom... Then to pin or dowel the center of the panel into the top and bottom rail. That way there is room for the cross-grain to expand. And the pinning keeps the panel from sliding inside the door (centered).
BUT-- Remember that the OP's medium for his panels is MDF right? MDF does not expand like crossgrain. Heck, MDF does not expand as fast as long-grain. Most doors using MDF panels do not require expansion joints. It would be better if it had. The joints usually do not pop apart, but some do end up rattling after age if you do not.
Another old-school technique I've seen on lower-end wood raised panel cabinet doors- is to give 1/16" extra on the sides, make the top and bottom snug. Then when you assemble, you center the panel in the rails, with just a dab of glue in the center of the top and bottom panel edges. This gives the same effect as pinning (not as strong, but). Some of these I've seen are over 100 years old and are still going.
The reasoning for the extra room is that cross-grain (like across a wood panel glueup has) expands faster, more and at a greater rate than long-rain (as with rails)... so if you get a big change in temperature or humidity, you pop your joint apart between the rails and stiles.
The reasoning for using something that compresses between the panels and the door's R&S is that if it does have extra space there for that expansion ... and if you don't have something there, then the panels rattle when closed.
The old-school of doing that without, or rather before people started using some kind of compression filler, was to give 1/16" additional for each side of the panel and none extra for the top and bottom... Then to pin or dowel the center of the panel into the top and bottom rail. That way there is room for the cross-grain to expand. And the pinning keeps the panel from sliding inside the door (centered).
BUT-- Remember that the OP's medium for his panels is MDF right? MDF does not expand like crossgrain. Heck, MDF does not expand as fast as long-grain. Most doors using MDF panels do not require expansion joints. It would be better if it had. The joints usually do not pop apart, but some do end up rattling after age if you do not.
Another old-school technique I've seen on lower-end wood raised panel cabinet doors- is to give 1/16" extra on the sides, make the top and bottom snug. Then when you assemble, you center the panel in the rails, with just a dab of glue in the center of the top and bottom panel edges. This gives the same effect as pinning (not as strong, but). Some of these I've seen are over 100 years old and are still going.
