MIMF

yea Barry, I was just sharing my experience with the teeter method . I did it one time years ago and would not suggest it to anyone ,it works, but there is no advantage to all the extra work involved.

Routing the slot, bad idea because you've altered the guitar permanently. Much better to modify the fret tang to fit the slot. Doing something that is simple without damage to the instrument. This is the crux of the original question...I'm amazed at all the comments about tension rods, etcetera, as if they relate to the original question.

For guitar making one should begin by selecting saws with kerfs that are closely related to the more common fret tangs available. Simple solution. Then if the chosen fret for a particular guitar doesn't fit it's very easy to make it fit....none of this should have anything to do with the tension rod.

If you intentionally rout the fret slots large in a guitar you're making you will never have frets that fit snug. This effects the clarity and sound. And in the future, refrets are a royal pain because some repair guy has to clean up all the glue. Plus, subsequent fret jobs could mean that the frets are not in their original place, otherwise messing with intonation.

Keep it simple.

Just a note Greg, LMII frets come from Jescar in Germany and you can order them in bulk directly from Jescar's division in the US. Three pounds of any size gets you a substantial discount. Most folks who've tried Jescar's fretwire won't go back. Many of the sizes come in multiple tang widths for refretting but LMII only carries a few.

Stephan, much depends on the fretboard material. I use African blackwood which is considerably harder than ebony, more like phenolic. It requires considerably more fret breathing room than other rosewoods.

David, you've just confirmed what I've been saying. Fitting the frets to the situation, i.e. harder wood is far easier, just takes a few minutes and prevents driving the neck back.
Oh, and thanks, I'll get some Blackwood!