Another way of looking at it is ...
Imagine lifting the entire valve section out of the instrument. Now look at the tubing that's part of the valve section and the tubing that's not.
In almost all cases, the tubing in the valve section is cylindrical. Even in the common case where the 4th valve tubing increases in diameter, the valve circuit is still cylindrical (doesn't increase in diameter gradually on its way to the bell). You can always test this by trying to turn around the valve slides and insert them "backwards". This will (almost always) work: they're cylindrical tubing.
But OUTSIDE the valve section the tubing is (mostly) conical. Of course it can't be truly conical in a tuning slide or that slide wouldn't ... er ... slide. So the legs in that case are cylindrical (but of different diameters) while the crook is conical. After the valve section, the tubing is conical and increases in diameter all the way up through the bugle.
In many (but not all) cases, the lead pipe (from the receiver to the valve section) will also be conical. On my 1924 Buescher, my Wessex Champion (Besson clone) Eb tuba, and my Amati oval euphonium, the lead pipe is obviously conical. On my Mack Brass (Jin Bao) euphonium, it appears to be cylindrical but in fact is conical as well -- the appearance being primarily an artifact of the relatively short length of the lead pipe in the classic euphonium case.
There are instruments that are fully conical through the valve sections (with the exception, of course, of the tuning slides). So far as I know, these are all tubas. I don't know of any euphoniums that employ a conical valve section.
In general, the run of tubing in a tuba/euphonium is conical (lead pipe)/cylindrical (valve section)/conical (remaining tubing through the bugle) while for a baritone it's cylindrical/cylindrical/cylindrical/conical (in bugle). At least as a rough approximation.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)