Traditional Irish Music
When the harmonica becomes dirty inside (dirty reeds), it might sound out of tune and become harder to play. At times, it might be construed as metal fatigue and this would mean tuning the reeds. I have some very old harmonicas, some aren't made by Hohner anymore and they still play as good as they always did. I found that completely disassembling the harmonica and washing the reedplates in a soapy solution IE: dish detergent in lukewarm water and scrubbing them with a soft tootbrush does wonders. As for the comb, a dry toothbrush does the trick. If the harmonica is held together with small nails, eventually, the nails won't hold it together because the holes become bigger with time. I found that using toothpicks to make the holes smaller takes care of this problem. If reeds need to be retuned, using a tuner, the pitch of a reed can be changed by either slightly filing the tip of the reed to make the pitch higher or slightly filing the middle of the reed to make the pitch slightly lower. This has to be done very delicately as there is no room for error. This is much cheaper than having to buy harmonicas. Another tip is to save old harmonicas so the reeds can be used to replace the faulty ones on the good harmonicas. This is what "Doctor Norm" prescribes for faulty harmonicas. Of course any suggestions are welcomed. At the price of harmonicas nowadays, anything that can extend their lives is welcomed. The best of the day to you my friends.
Interesting topic and one I had to struggle through myself as well. IMO every harmonica player should know how to gap and tune reeds as a bare minimum. Luckily for me in 4 years time I have yet had to retune because my harmonica is out of tune. Maybe this is because I've mostly been playing Seydels with SS reeds, or maybe I'm just careful while playing.
I've retuned all my harmonica's to Paddy Richter tuning using the technique you described. I've also had a phase where I valved my diatonics, while this does allow for more expression it also gives more hassle. Sometimes the valves get stuck in the middle of a song making the harmonica less reliable. Right now, I'm neither a fan nor an opponent of (half) valving the diatonic harmonica.
As for cleaning, most of the dirt piles up near the entrance of the hole, usually I can clean the harmonica by just picking out the dirt there. I try to avoid washing personally, the combination of metal and water just don't go that well for me..