I got my first couple boxes of Dominion a few years ago now, and over the last few years worked up to the eight I have now and I have my eye on the remaining few. My older sets (especially base cards) are rather noticeably more used than my newer sets. Recently I've been liking the idea of sleeving my games. Should I bother going back and sleeving all my Dominion cards even though some of my cards are starting to be visibly aged?
You may be able to find Euro-sized sleeves with opaque backs somewhere. Then it wouldn't matter if some of your cards have noticeable wear.
If it's mostly just your base cards (Copper, Estate, Curse, Province, etc.) that have noticeable wear, you could get a set of Base Cards. If you have the first edition of the base game and all of its cards are looking old, you could get the second edition, which comes with an (admittedly super thin) trash mat, better card wordings, and larger font sizes on cards. If you do any of this, I guess it's up to you whether you want to invest in sleeves. I did (I use penny sleeves) and have no regrets so far.