This issue was indirectly raised in a social media group I belong to. Seems some think they shouldn’t charge, but often without realizing why some of those authors and celebrities charge. After seeing too many times where rude, pushy, demanding, and sometimes hostile fans have treated authors and celebrities, I don’t blame those who charge. Some of them donate the money to charity; others charge to reduce the privacy invasions that have become rampant lately. That’s not including the autograph seekers who want your autograph so they can sell it to the highest bidder on EBay or some other site. One solution to reduce this group is to make the autograph personal. An autograph to John Doe is worth a lot less to collectors than say an autograph to a well-known celebrity, like William Shatner. The personal autograph to Shatner would probably increase the value a lot more to Star Trek fans.
Should you become a well-known or popular author or celebrity, then you may face a similar situation. I say go with what works for you, but be willing to adapt if you later need to re-think your earlier decision. Also, don’t use your regular signature, but create an unusual or unique one for your fans. Too many places online will accept a digital signature as proof that the signature is really you.
I have seen some authors and celebrities actually create a rubber stamp they use for a signature. In those cases, it tends to be a made-up signature most of the time. In some cases, an author at a book-signing will sign hundreds if not thousands of books. Sometimes they sign them ahead of time to avoid having their arm go numb from the signing.
When I was younger, I attended science fiction (sci-fi) conventions. I got to meet and talk with Richard Hatch of Battlestar Galactica (the original series and the first re-boot) before they made the second re-boot. At the time, he was trying to get the series back on TV so guessing late 1980s or mid-1990s. Back then, most authors and celebrities didn’t charge, but the fans for the most part respected their space. These days, many fans have no problem violating personal space and boundaries.