Should v sign?

Whenever v swipe a credit card, v r asked to sign. So, now comes the ridiculous question, y v sign?
Well, how did i run into this Q? I signed a paper for my credit card swipe & the shop keeper took it. But, later i asked him for a credit card statement bill & all he did was, returned me the paper i signed. Doh!! I cudn't beleive wat i'm seeing. I q'ed him & the shop keeper is like no worries. u keep ur signed paper.
Now, at this point...Do v really need to sign the paper after the credit card swipe. Luks like it ain't reqd no more. But, where is the customer gaining vantage? Well, later the customer can always decline to the credit card comp saying tht the amount charged to his/her credit card is not a valid one & the shop keeper won't be able to prove, w/o the proof of the signed paper.
General policy for securit - V better be signing the paper if v r asked for. V also better be checkin our credit card statements to be sure tht v r not being over charged or tht ours is not being misused!!