Brian has literally written the book on this subject, so, as always, his advice is to be taken very seriously.
It really depends at what you find easiest ranging with your eye and with some help from the scope. As Brian said, if you have low or medium mounts then the closer targets will all be covered by just a couple of aim points. Remember all of the 15mm kills are between 13 and 25 yards. So most people go for lowish mounts so they can pretty much use the same aim point for the 15mm kills.
If you find ranging the closer targets ( up to 30 yards ) quite easy and you can do that within a couple of yards, then you may find it better ( for you ) to have higher mounts. Using the higher mounts you get the impression that your trajectory ( with respect to aim points ) is flatter at the longer targets. So you will only have one or two aim points at the longer targets. So if you have a problem ranging the longer ones ( say 35 to 45 yards ) then higher mounts and less aim points over those distances may help you.
Some will say that the kills are bigger on the longer targets so it's not that important. Kills are now 25mm out to 40 yards and a lot of UKAHFT 45 yarders have 35mm kills. So it's all relative.
Personally I found I was very good at ranging by eye, with some scope ret bracketing help, out to 30 yards. So I was confident that I'd range the very small 15mm kills to within a yard or so. Once you can do that you only need to know your aim points for the close range distances and you'll get all the 15mm kills. You don't have to know aim points for every yard or be able to guess every target to within a yard. Just a few different aim points will cover all the close range distances. So I moved to high mounts and just worked out the aim points for the closer targets. The long stuff then became so much easier ( for me ). I only had a couple of aim points at the long stuff and I could get them all inside the kills on the ones I wasn't too sure about.
Some shooters ( a lot of FT guys ) think the vertical balance of the rifle is very important. They like the bore ( centre of barrel ) to be in the centre of balance of the vertical range of the rifle set up. In HFT most shooters will use a hamster to give extra depth to the underside of the fore end. This helps with low HFT style prone and also with kneelers if the fore arm is being rested along the thigh. So if you use a deepish hamster but you also use low mounts, then the bore is quite offset toward the top of the rifle set up's vertical depth.
Here's a photo of Dave Ramshead taken just a couple of weeks ago. He's as good as it gets when it comes to HFT. He's cleared a UKAHFT Gathering course ( more targets than a normal course ) and he's been National Champ on more than one occasion. Check out the mounts on his rifle and check out the height between the bore of the barrel and the centre of the scope. It even looks like there are riser blocks as well as the mounts. So not everyone who is a top shot at HFT goes for low mounts.
Maybe worth noting that Dave has his gloved fore hand lying on the floor but with the back of his hand in contact with the floor and the rifle rested on his upfacing palm. That will only give a shallow depth of his hand between the floor and the rifle. So, to keep a comfortable neck and head position he may need higher mounts to line the scope up with his eye ( he could use an even deeper hamster but there is a limit to the depth you have below the bore ... 150mm ). A lot of shooters have their gloved hand on the floor with the back of hand and palm at 90 degrees to the floor. So the padded bit from the wrist to little finger is in contact with the ground. The rifle then rests on the area between the first finger and thumb. That hand position gives far more height so the rifle will be higher, so the scope may have to be lower to give that same comfortable neck, head and eye position.
Like all things HFT ... you get started in the sport and start enjoying it ... then you experiment and find out what suits you best.