Since I've been practicing full time, I've noticed that I need to wipe my guitar clean at least twice a day. It's not so bad now that the weather is cooler and I don't break a sweat sitting in one place anymore, but I still get crap all over my guitar even with clean hands. Just the sweat and oils from my hands leave residue wherever I touch it. I never used to think much about this, but it's obvious now that I play so much.
String cleaning is most important, as it affects the tone, playability, and life of the strings. I find that a retired pair of boxers (washed, thank you) with the elastic removed makes a better lint-free cloth than most that are available for purchase. When I clean my strings, I use my fingernails with the cloth to get underneath each string and scrub it over its entire length. It's easy to feel the sweaty buildups through the cloth, so I keep working it until it's clean. The unwound strings collect the most garbage; I rarely notice any buildup on wound strings.
I also need to wipe off sweat marks from the back of the neck and the body around the bridge every day. It's far easier to wipe these off on a regular basis than to wait until it's all caked on. Using extra force, scratching, or chemicals to remove old residue increases the risk of damaging the guitar. On a recommendation from Dan Erlewine's Guitar Player Repair Guide, I find that the best chemical for wiping away minor buildup is my own saliva. This just keeps getting better. I spit on my guitar and wipe away a dried skin/sweat/oil cocktail with an old pair of undies, and it's as good as new!
I clean the whole fretboard every time I change strings. I'm a heavy user of GHS Fast Fret. Its selling point is that it cleans strings and lets you play faster, but I like it most for fretboard cleaning. I use an old school Bic mechanical pencil through said special lint-free cloth to get any dirt out of the corners by each fret. This is also a great way to clean the bridge without taking it apart.
I don't mind dents or scratches at all if they don't affect my guitar's tone or playability, but I've realized that keeping it clean is essential for both.