Dipping

The only clear-cut answer to your question is -- it depends. Ugly, unattractive toning would be the only reason I would even consider it and then, it depends on the coin. I've dipped some ASE's that blackened due to improper storage, but I would never attempt with a high-end key date.

As already mentioned, coin dip is acidic and removes small amounts of metal along with the undesirable toning. On an uncirculated coin the first thing to go is mint frost or luster. Over-dipping removes all the frost and you end up with a lusterless coin. We call a coin overdipped when we see one that is obviously uncirculated or high AU and it looks flat and no remaining luster, even in the recesses.

In order for us to give the best advice we would need to know (or see) the coin you are considering. Blanket approval/disapproval is irresponsible from a numismatic perspective.

One of the biggest mistakes dippers make is not rinsing the coin after dipping, leaving acidic residue on the coin which will form spots over time. Another major mistake is dipping a coin with a very heavy patination, the removal of a thick oxidation layer will leave a coin with dull surfaces and no luster.

Not every coin with toning/tarnishing/patination is a good candidate for dipping. Many more coins are ruined by dipping than helped by dipping simply because the dipper did not properly choose a suitable candidate. The best candidates are uncirculated coins(gold, silver, and nickel alloys but not bronze/brass alloys) with light-medium patination. The best dip is the shortest one possible to remove the offending oxidation or fingerprint, longer is most definitely not better. Once a coin is dipped, it should be thoroughly rinsed with clean water(running tap water works fine) followed by a distilled water rinse and then acetone to remove any residual water.

Since the bulk of my collection is early or branch mint gold, I avoid dipping at all costs. Dipping gold generally doesn't provide beneficial results... very risky to do as it tends to "brighten" the coin too much... yuk.

I prefer my gold with a little dirt anyway.

ET

It's a crap shoot. I have dipped ugly toned PF-65 CAM Morgan dollars. One came out so nice it went into a PCGS PF-65 DCAM holder. The other ended up in a PCGS PF-65 holder. My technique is to dip as fast as possible and immediately rinse with DI water.

It's basically how lucky do you feel.

What I have found interesting is that dipped coins tend to retone right back the way the way they were prior to dipping over time. Not exactly sure why that is.

As a master machinist and life long metal worker I know this to be true. Toning and oxidation actually acts as a protective layer keeping the metal underneath the oxidation from corroding any further. By stripping away the oxidation you allow the fresh metal underneath to corrode as well, resulting in the drab over dipped coins we see all too often on the market. I personally would never dip a coin but to each their own.

The main problem here is it is not clear as to what a coin should, should not be dipped in. Dipping in distilled water, for example means little. Dipping in Acetone too means little. Dipping in strong acids or jewlery cleaners too can usually do some damage. Dipping a coin in all sorts of things people find to dip a coin in could be bad, very bad, have no effect.
The best thing is to just leave a coin alone.

Cleaning is anathema in numismatics. This is a large hobby, though, and like other endeavors there are outlying cases which deserve further scrutiny and discussion. It's the nature of a Forum like this that we'd end up spending disproportionate time on the outliers, because a positive decision to conserve (note my use of the term) still invokes a "do the least harm" protocol. We need to clearly delineate this process for all reading, so we tend to be verbose on the subject.

So don't clean your coins.