...is a question many have asked here and elsewhere over many years. Here is a little fresh insight in pictures.
Every day after practice I use the BlowDry system to remove moisture, then I clean and oil my valves. The next day I just pick up the horn from my Hercules stand and start to practice.
Here are 2 photos from today, showing my 1 & 2 piston tops. I chose this view because this is how my pistons pop out when the horn is sitting sidewise and I open the caps. It's a bit hard to see here, but it IS significant, I think. In any case, I see something like this every day. The small, very soft deposits didn't come from nowhere! I have to assume that they are a result of oil trying to mix with water, more or less. I put a lot of moist air into a horn, so maybe my valve oil is more challenged than for most folks.
But it makes me very glad I'm now cleaning the pistons each day. Overall, this is the best pattern of care I have ever followed, in terms of results.