Maybe it makes a difference if the template/pattern/master (seems to me we had a discussion about terminology not too long ago) is internal or external? The patterns I made for my turtles were made of 1/2" MDF, but I screwed them to the blanks and used a top-bearing pattern bit to trim them to finished size - I would agree that the thicker pattern may be a better choice in that application. However, I think that 1/4" hardboard or MDF is good for an internal template where the template guide is riding along the inside and, more importantly, there is usually much better support for the router base - although a it's probably a wash in either case if an extended/oversize base is used on the router.enough to give me an adequate lead in to the notch.
Similar to how I do mine, except my masters are 1", and nailed on. My router is in the table, with a top bearing bit. I use the 1" masters for inside routing also. Use the master, mark the inside -of the blank, drill a hole or two, rough cut with a sabre saw. Tack the master down, and have at it. The 1" master not only give a better grip, but I can buy nails just long enough that I can smack them flat on the master (not usually done)I, and they will still not penetrate the blank, quite. The 1" also means I don't have to be so precise on where the bearing rides on the master
I managed to dig up one of my old masters, that was used for routing inside a blank. I don't find it any different from routing the outside, just a couple more steps first. Works for me, and like I said, been doing this stuff for years, and no issues. Oh yes, I also put any pertinent information on my masters - the reason it says flip, is because these are outside pieces, so needed a good side of the plywood on the outside, hence the flip. You can clearly see the nail pilot holes drilled in this. Many more than actually needed, usually, but every once in awhile the nails will start dancing out, which means not enough nails - sometime just whacking them flat works, but usually just use more nails, then it's easier to pry the master off, rather than working on fully driven nails.. And no nail holes show on the finished whatever. Never tried screws, and don't plan to.
"It ain't what you're told, it's what you know." - Granny Weatherwax
Fawkahwe tribal police SWAT Team
Some days, the supply of available curse words is insufficient to meet my demands.
.....Call me a craftsman, artisan, or artistic, and I will accept that. Call me an artist and you will likely get a quite rude comment in return. I am not a @#$%ing artist.