personally and this is just that MY 2 cents
I think most mobile mechanics, appreciate having a clean machine to work on, and in a place where its easy easy to work on as possible,l, as in NOT in mud, or off camber ground, rocky ground or just wet, near electrical outlets(if needed) GOOD lighting and if in a cold area, a warm place to work on it!
the reasons I say this is, well time is money, the more time it takes for them to find a leak, due to having to clean things to find exact location, or time it takes to just clean enough to get to things, same goes with having good light to see, and , well, the more comfortable the tech is, with the machine being in a easy to work on area(not all muddy wet or rocky ground)
the happier they tend to be and that tends to mean more productive in my experience
showing up to a sloppy mess, doesn't inspire anyone IMO
and also, having a clean machine for them to work on also means there less likely to end up getting dirt or crap into a CLEAN hydraulic system, when they remove hoses or connections, which benefits YOU more than them in the long run!
SO< yes if possible, wash the machine, let dry and then call them to come check things out
make there life easier, and they will be more likely to want to work for you in the future as well, which might me less down time for you!
so thats my 2 cents