Nice video, and a good tip!
There's quite a bit of variation amongst instruments in terms of the threading, the quality of the threading (depth, uniformity, precision), and the hardness of the brass. This can translate into some frustrating differences.
I NEVER have had any problems in cross-threading the valve caps on my (1960s) Amati oval euphonium or my 1924 Buescher tuba. The threads are cut very precisely and definitively, with good depth. The (1990s) Cerveny rotary tuba I had sometimes gave me a little struggle, but nothing too severe.
Now comes the Mack Brass euphonium, the Wessex tuba, and the Schiller trombone. Much greater care must be taken and the risk of cross-threading is substantially higher. It's all because of the quality of the threading and (to some degree) the fineness of the threads. The Schiller is actually (and suprisingly, to me) the easiest of the lot. In part, this is because the rotary valve caps are just "right there" and easy to get to and to get on. The Mack Brass (several years old now) seems to have the softest brass, but the Wessex (now also several years old) isn't far behind. And the threads on all of these are pretty fine.
In my experience, finer threads give me more problems in terms of initial alignment and getting started. The Buescher and Amati threads seem definitely coarser. Dave also demonstrated his technique only on the three top valves. The forth one tends to be even more problematic (for me, at least) unless I put the instrument on its side so the valve is oriented vertically.
Finally, while it helps to put some oil on the valve cap threads, I've found that something a bit thicker works even better. When I run into any problems with valve caps I put a small bit of Dow Corning High Vacuum Grease (silicone) on the cap threads. From the beginning, I had a problem with the second valve cap on my Wessex tuba threading on and seating correctly. Jonathan even sent me some replacement caps, but they didn't improve things. On magnified inspection it appears that the threads were just somewhat poorly cut on the valve casing. Using the silicone grease made it work well enough, and over time seems to have resulted in improvement to the point that I don't have the problem any longer. Sometimes, if threads are cut a little sloppy or with worn tooling, the pitch, thread angle, or depth will not be uniform or the thread depth will be too shallow (sometimes just in places), and this will make correct thread engagement more difficult and sloppy. In such cases, using a grease of some sort as a void filling agent can greatly improve the function of the threads.
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)