I am playing Eb tuba with 4 valves (4th at my right hand pinky). Usually I have my fingers more or less straight out over the valves, resulting in the tips for all other than the pinky is sticking out a bit on the other side. Sometimes when I am playing some technical demanding stuff, I try to curl my fingers a bit so the finger tips are pointing a bit down touching the valves. Psychologically, this feels like I play a bit more precise. Is this a technique that is known to be better, or is it just me that think so?
From an equipment standpoint alone this is a better technique, and is encouraged whenever smaller piston instruments are taught as well. Curving the fingers on top of the finger buttons ensures that the force on the valve is going to be straight up and down instead of to the side. On student instruments, this is especially important since seldom-oiled valves are likely to get stuck if force is coming at them from the side instead of straight down.
With larger valves like tuba and euphonium, this is a little less important, but the concept does still apply.
As far as the physical technique of hand position is concerned, you should examine which muscle groups are active and how much they need to move for each method. The knuckle joint is going to be doing most of the work in either case, but if the fingers are curved, the movement necessary is going to be smaller than if the fingers are straight. Also, if the fingers are straight, you are tensing the second and third finger joints unnecessarily. (Unnecessary tension = bad.)
Depending on how your valves are set up, you will probably need to be a bit more extended than you would on say, trumpet, but curvature of the fingers to the degree that the pads on your fingertips are the ones that are actuating the valves straight up and down is highly encouraged.