Keep in mind that there are more than a few collectors who consider chemical cleaning of stamps a form of alteration, and that doing so without documenting it as unethical. Not everybody shares that view, but it is not a insignificant number. There are also plenty of major dealers and experts who do it frequently. There are different "levels/degrees" of cleaning, which makes things more murky.
A major problem is that fraudulent sellers often use chemical cleaning in their methods of fakery, thereby causing problems for those who are trying to preserve items in their collection or are simply innocently making a few of the their stamps better in appearance.
Whether one considers it ethical or not, favorable or frowned upon, will always be a matter of personal opinion (that's my take).
It's pretty easy to forget having cleaned/altered some stamps. My point is, the problems occur when somewhere later down the line, someone gets accused of dishonestly selling altered stamps, even though there may not have been any attempt at fraud. So document any physical alterations you make to your stamps/collection. All it takes is for someone to spot one altered stamp and the buy offer for your entire collection can immediately drop 50% or even withdrawn. Most buyers will not sit there and examine every single stamp in the collection -- so once they spot a major problem stamp, the assumption will be that there are other problem stamps scattered in the collection.
Just my thoughts! Enjoy your stamps, they way you want to -- but don't be surprised later on. I still remember one collector who meticulously cleaned and repaired a large collection of mostly common US stamps. I admit I was a little dumbfounded at first and just kept my mouth shut and observed -- he loved every minute of it, had no plans on selling or giving it away later on -- just wanted to have fun.
