It all depends upon why you're getting a book signed, and what book it is. And maybe the question should really be is it worth risking a grade drop to get a signature, because in most cases a book will keep the same grade, especially if a SS witness handles cracking the book out. The SS witnesses handle books just like their own, so if a book gets damaged in the signing process, it's generally because of something the signer did, and not the witness.
If you're getting the book signed for yourself as opposed for resale, then in most cases, it's worth taking the small risk, because it's really all about the signature and not the grade. And even if you are getting the book signed to sell, it's probably worth the risk because it's so small. The exception is when the difference in price if the book drops is significant, and if you're talking about copper age and modern age books, in most instances, especially with moderns, the difference between a 9.8 and a 9.6 is not that much, mainly because a 9.8 is not worth that much, so there's only so much the book can drop. G.I. Joe #21 is an example that comes to mind, and even on that one, the difference between 9.8 and 9.6 is not nearly as much as it used to be. Once you move into the bronze age and older, the differences between grades can be much more significant, such as Hulk #181, where a 9.8 goes for $15K plus, and a 9.6 can go for less than $5K.
Bottom line is, if in your mind it would be a disaster if the book drops in grade, then don't get it signed because nothing is worth that kind of stress. On the other hand, if you really want the signature and don't care if in the unlikely event the grade drops, then go for it.
Just my