Hi Johnno - You raise an interesting question there - and one I will follow with interest - as I wonder also what oher people's views are.
Well - to start you off - here are some of my thoughts about this question.
Although I am not a collector of the figures you mention - I guess they will apply to any manufacturer. The ones I know best are Britains - so this is what I think.
Firstly, I regard myself as the temporary custodian of my finds - some of which may be older than me ( and that's saying something!). Now I'm a painter - of sorts - and love to get my brush to work on most things that come my way.
BUT - I'm a bit more cautious these days - before I start. First thing to assess is the rarity of the piece. Is it a rare example. Also, is it complete (e.g. any limbs missing). What is the paintwork like - i.e how much missing. I think that if it was rare and had a fair proportion of its paint intact - I would tend to leave it alone. Some future collector may not bless me from slapping a new coat of paint all over it - or even touching it up a bit (CAN significantly de-value a piece).
On the other hand - if (as happened to me recently) a figure turned up - about sixty years old - with no head - missing one arm and lance - whilst the horse had two legs missing - and couldn't stand up (answered to the name of "Lucky"

- well - no problemo - a repair and repaint was called for. In other words - if it won't stand up and looks awful - I think the full monty is called for in that case.
Another example: I recently picked up a Britains Madame Tussauds souvenir figure. Sort of rare - made 1935-ish - intact but with worn paint. I was tempted to re-paint - but held back. Its old enough and not many are about - so to a future collector - it could be a nice piece to own in its original (not bad) state. Now I don't collect as an investment - I never part with any of mine anyway - but value needs to be considered too. A re-painted rare piece can be made almost worthless by repainting.
I left it alone. He's in with my new stuff - and looks a little shabby - but is growing on me as an original piece, and - I think - a nice thing to have in my collection.
Last thing is though - YOU have paid your money - and they belong to you - so you CAN do what you like with 'em. BUT - neither you nor I will be around forever - and the ones you recently bought were owned by somebody else once. I always say a little "Thankyou Pal" whenever an oldie turns up who has been cared for by someone else in the past. I hope they can hear me - and I hope whoever gets mine when I have gone, will think the same about me.
In other words - just use your common sense in assessing whether to re-tuch - or repaint. If in doubt - leave it alone for a while - and re-think it over - and decide what to do later.
Hope that starts the discussion off okay - and hope that helps 'cos that's what I think. johnnybach