I don't see a problem with cleaning.
It's very common with museums. The LOC has some excellent articles on specific conservation projects they've done.
http://www.loc.gov/preservation/conservators/
They go much farther than most collectors would. But that's not unusual. In some fields it's almost required. Many old posters are preserved by backing them with linen attached with a rice paste. They're simply too fragile otherwise. LOC also does deacidification to slow the degradation of stuff on acidic papers. (They even tried building a machine to do books in bulk since their collection will take so long to do some of it will be lost)
I'm not sure about alterations that go beyond cleaning and preservation when it comes to cards.
Many of the strip cards probably should get some treatment, deacidification for sure for the ones that aren't already brittle and coming apart.
Removing creases and trimming- isn't ok for cards, especially in a collecting context.
Rebuilding and repainting- I don't see it as appropriate, although in extreme cases it might be. The Wagner that was reconstructed the un done was a maybe. And large display pieces perhaps should be restored.
Disclosure is always an issue. Most museums etc now keep careful records of what was done and how. But that wasn't always true. I believe most very old paintings on display have been cleaned. There's been some debate whether it should be done for some. One side wants the painting shown as it was when new, the other prefers leaving it in a preserved state that keeps the original colors muted as we've come to expect to see them under 300+years of dirt/dust from the air.
A straightforward light cleaning will pass SGC and probably PSA.
Here's the before and after of the one I did. Water only, Q-tip and a soft cloth to dry with. probably close to 100 years of dust and soot. The fine cracks in the papers coating wouldn't come clean without more extensive efforts.
Before
After
Slab aside, which card would you rather own? Who would leave the card dirty, knowing that the dirt would probably cause further damage with time?
Steve Birmingham