I think like so many issues in magic and mentalism, there are times where doing something one way is right.
In the case of signing things for mentalists. Having someone sign a seal on an envelope adds to the authenticity of your claim that you could not have tampered with it.
In the instance of borrowed objects, coins, bills, etc... with coin bending for example, the question is 'if its not signed will the audience suspect a switch even if I do keep the handling very clear?'
I have a serial number reading trick in which the borrowed bill is folded up by a spectator and then across the two visible sides he signs or makes a mark. The only time I take hold of the bill is at fingertips when I place it against my forehead and call out the numbers. The bill is handed back folded in eighths, the signature confirmed, and then unfolded, etc...
I tried that effect without the signature, despite the fingertip holding of the bill, everyone swore 'he switched it'. So in that instance, even with an object that never leaves their sight, it has to be signed.
That serial number trick for those interested (at the risk of this looking like one big cheap plug) is called \'The Dead Zone\' and appears in issue 2 of The Foo Can
The best spirit slate routines have the slates signed in advance of the writing appearing. But if two slates were clearly laid on the table having been shown cleanly on both sides and the magician never touched them - surely the signing in that case is redundant.
So my guess is that it depends on the effect as to whether the item has to be signed. Going back to that original coin bending example - Derren Brown does a coin bending but does not have the coin signed or marked. But thanks to his verbal control, and the effect he does first, and the way it appears to all happen in someone elses hand, means signing or marking would do nothing to enhance it.
OK, now I am rambling! I'll stop.