It's kinda like most things where there's not really a "what works for one person, works for everyone" deal.
And that's not necessarily to say that there isn't a set "right or wrong" way to go about some maintenance duties but that's all subjective and since we're all different, we'll go about tackling things in different ways.
I personally allow some things to go until there is a clear sign that I need to ( or should have) address(ed) something. But I start with inspecting and checking anything that rotates, pivots, compresses, slides, etc.
I don't necessarily keep my rigs super clean but I try to make it a point to perform basic/ quick inspection and maintenance. Differentials, portals, etc for example... I don't feel the need to disassemble often but I do like to make sure that they are moving/ feeling/ sounding the way that they should. If certain parts feel gritty, are binding, or have excessive play, then it's time to get em opened up.
Again, for me what works is inspecting critical moving parts- linkages incl servo horn, servo mounts, control arms, wheels, driveshafts, axle shafts, etc.... anything that with failure will leave me carrying home a heavy rig. Other parts should also be inspected but may not be as critical or may not typically need anything done- motor, axle gears, shocks, etc.
I've never flushed or sprayed my rigs with water but I also don't typically run through deep mud or water so I don't feel the need to go that extreme when it comes time to clean. Some people use a hose or pressurized water source and swear that it works. But I simply don't like doing things like that and I don't like the idea of forcing water ( and debris) into tighter places that may take a long time to dry, or may be inaccessible, or that may be more sensitive to water and subsequent rust. Same with my tire-foams... I don't like them to get wet and/ or have to wait to dry... as well as going thru the hassle of disassembling and reassembling wheels when I don't have to.
My most valuable tools when it comes to cleaning are a couple cheap brushes, small stiff brushes, q-tips, thin rags, paper towels, WD40 or similar spray like engine degreaser/ brake cleaner depending on the specific part(s), Windex or whatever water-based cleaner, and compressed air ( although I use it very sparingly and not on all parts... same mindset that you wouldn't use compressed air to clean out earbuds). But I take my time because I enjoy these aspects of RC just as much as actually running my vehicles. Of course, not everyone likes maintenance tasks so they aren't going to want to do it like I do.
However you decide works best for you, just make sure ( especially if you're blasting the rig with water), that you are also allowing things to dry thoroughly and greasing/ oiling/ lubricating afterwards as well as applying fresh thread-lock where needed. And always a good idea to double check radio/ receiver/ esc functions after major maintenance, adjustments, new parts, etc.
Before hitting the trails, my piece of mind also comes from taking along a small ziplock bag with some asst hardware ( like a couple wheel nuts, body clips, xtra servo-horn screw, etc) and the appropriate tools to do those repairs if need be. Crazy how losing one little nut or screw can ruin the whole day.