It depends on the card, I think. If you had a Joe Balogna card and had it autographed it becomes a piece of memorabilia, therefore worth more than the $.02 book value. But, if you took say, a Honus Wagner signed card, it may be worth less because it's a rare card that's defaced. Plus, you have to be able to PROVE that it was signed by that player.
Ultimately, who cares? If you're having fun with the hobby, cards, and like the players or team you write to, it shouldn't matter.
That is a matter of great debate and opinion. As stated, historically significant and expensive cards tend to be frowned upon as autograph mediums, where as newer cards, collectors issues and those with lesser values tend to be more popular. Not to mention the risk involved in sending off cards, unless you are obtaining a signature in person.
I personally would get any card signed and the more significant or valuable, the better. I'll use a 1949 Bowman Duke Snider as an example. Snider has signed TONS of autographs in his day and continues to do so. I bet there are more 1949 Bowman reprint cards signed today of him that you'd ever imagine were possible, yet few are willing to get the real deal signed. I personally hate how many 1990 Pacific Legends and Swell cards are floating around out there, when they could just as easily been Topps or Bowmans! Sending it to Snider's home through the mail would be extremely risky. Sending a 20 cent reprint is not. If I ever had the chance to meet Snider and get any cards signed, his RC would be my first choice. Some reasons: Few get signed so it's scarce in that regard. Arguably his best card. Finally, replacing a Bowman Snider, albeit expensive, is easier to do than a signed one, especially after he passes away.
You may not see a return these days from a vintage signed card the way you would an unsigned version, but I suspect that only really applies to the big money cases when the card is in pristine condition, where an extreme premium is added for condition. Another reason why you see so many beater cards that were signed.
In my photobucket, you'll see many examples of vintage (and valuable) cards that were signed. Decide for yourself what you'd rather have in the end...another 1977 Renata Galasso collectors card or a nice 1954 Bowman!