First off, welcome to the Forum!
Second, I'll admit that I've never cleaned the inside of any of charcoal grills. I will also admit that I'm not the guy who uses his grill every single day (although I would love to become that guy, it's not practical for me right now). In fact, if I grill or smoke something more than two or three times a month I'm doing pretty good (sad I know, but true). I guess all that to say that my grills don't get caked up with grease etc.
Anyway, my normal routine is to simply scrape the cooking grate(s) clean and get all of the used charcoal and ash out after each cook. I also tend to take a paper towel (or wad of newspaper) and brush down any ash in the bowl before I start cooking. I figure the less ash before I start the less ash to swirl around and get on my food. I have recently started either wiping down and/or using a grill brush to get any grease flakes off of the OTG lid. But in my defense, it took a few years before the buildup started to flake. And even then, I've really only seen the flaking on my 22" OTG, which I suspect is because I basically never use a water pan in the 18" OTS so there isn't that much moisture to begin with.
To be honest, I've never really thought too much about cleaning anything but the cooking surface until I got my 18" WSM. I'm still trying to settle into a cleaning routine with it, but I'll probably start having a Weber Cleaning Day every year where I do a good cleaning of all of my Weber cookers. Well that's my goal anyway.
So, to sum it all up:
1 - Welcome.
2 - I haven't done much cleaning (other than the grates) in the past, but hope to do better.
3 - Unless you're starting to have lots of flaking issues, or are concerned about grease fires, I probably wouldn't worry too much about cleaning.