I think it goes both ways. For a fixed blade I usually start with 3/8". That give me room to shape and contour depending on what I am doing with the blades handle! I would have some to grind off, than to realize I don't have enough thickness to shape it like I want! Now if I do something like this Moran style handle I want to go even thicker to be able to flair that butt!
On something like a large folder 1/4" will work or thinner! On slip joint pocket knife you can get down to 1/8". There is not a lot of contouring on small folder!!
But as a general rule, I alway like to have more than I am gonna finish out to!! You can always take more off but if it is too thin when you get it fastened to the blade to contour like you want well, ............... all you can do is cuss! That don't help as my wife reminds me but, it makes me feel better!! LOL