I'm lazy myself, and furthermore don't have the skills to disassemble, clean and reassemble a typewriter. Probably I should, since there's a good possibility it'll make the machine work better -- which is the most important thing.

I do try to clean it up to the extent I can -- shell, keys, typeslugs, platen; blow out cobwebs, eraser crumbs and the like. But that's about it. I am not aware of there being "concours d'elegance" types of typewriter shows, as there are with collectible automobiles!

In terms of cleaning affecting value, I will start by saying that typewriters are products that were made in large volumes -- these are not pieces of early American furniture, say, which were handmade and for which a cleaning drastically reduces their value. So I'd say cleaning can only benefit a typewriter, both in terms of its usability and its value. But again, typewriter values are low to begin with, so go for it.

The big exception is for very rare variants, or those with a special provenance -- like if you can show that a certain machine was actually owned and used by a James Baldwin or Tennessee Williams, for instance. In those situations, especially ones with famous owners, I think you want to leave them alone -- don't remove the great man's fingerprints or nicotine residue.

There are plenty of points on both sides of this question, and for a machine which exists in reasonable numbers, I can see no reason why the owner should not please themselves how far they take the restoration.  If it restoration in the true sense and not what some in the antiques trade call restoration, which means stripping down to the base metal anything with a brass base and polishing it until it hurts to look at it.  That sort of vandalism is common with vintage telephones and navigational instruments, for instance.
I think a machine should be basically clean - the way its original owner might have cared for it.  But super-bright chromium and immaculately repainted (usually in inappropriate colours) in the fashion of vintage car bibbers is a sad case of more is less all round, IMO
However, typewriters of rare make or model should, I believe, be treated as living history; no restoration that removes or covers up permanently any original finish is acceptable in my book.

As an example of this, I have an Alpina SK24 from the 1950s which is, as far as I can tell, unused except by me very occasional and carefully, and the original finishes are in virtually mint condition generally.  I shall not touch them.  There are very mild traces of surface rust to one or two linkages internally, and I have simply covered them with a light grease to keep out any air or moisture, so they are now stable, I trust.
The only other thing that I can think to say tonight is that it is doubtless wise always to stop short in the restoration, and err on the side of preservation than go to far in making things look artificially and unconvincingly new.  Good luck whatever you decide to do while you are guardian of this thing from the past.

It has often been said that a fine watch works best with tampered with the least.  A typewriter essentially has clockwork innards, and to try to take one of those machines of 2,000 or more parts and to try to put it all back together again in my opinion is nutty.  Even if you could manage to get the whole pile of levers, keys, type bars, gears, rollers, and bellcranks back together in one unified piece, the machine just won't be the same, whether you've got it together the right way or not.  NDW76 is right:  It would lose its soul and character.  And the "old typewriter smell" would be gone (I almost cried getting out that one).

Heh, I usually always mostly rebuild mine. If they are easy, like old royals, underwoods, anything older than the 30s. Desktops of course. I'm going to try dip dunking my first tomorrow, an underwood 3 bank. No it is not needed for most people, however if you sell them like me then yes it is. It makes the typewriter much more clean then just bowing out air (which is still perfectly fine for most) however for that complete clean you have to take them apart at least some of the way. For example with an underwood 5 I take out the type bars and the carriage the ribbon advance mechanisms, everything but the keys and clean it. It makes a huge difference because I am able to polish everything in the hard to reach areas. Then when you put it back together, it's great to adjust it to your needs. Some you can't do this with though, like with a noiseless. That's when dip cleaning or whatever else works.