This has always been one of those more contentious issues. Everyone likes free autographs. "Back in the day," before conventions became more popular and whatnot, this was the thing for ttm collecting. People discovered who signed for free and then inundated them with items.
There are the "purists" today who are insistent that they're entitled to free autographs and just because they send a request it means they're the only "true fan" of the actor. I say that facetiously, because times have changed and the etiquette of fan mail doesn't mean as much as it used to.
I've actually handled fanmail for Terry McGovern previously at his request. I was stunned by one "fan's" request that Terry sign 13 index cards, no personalization but "they're for my family for Xmas presents." Yeah. Right.
I think if you're requesting items ttm, two photos and an index card should be a good maximum cut-off point for a typical request. If you're sendin' a pile of items with "no personalization" requests, most of the celebs can see through it -- and it does affect the fan who's merely asking for their own collections. They're way more savvy realizing that people will line up to pay for their signature at a show and it's easy to see when someone's looking to profit: I know, I've read the fan letters.
When I did send ttm years ago, I typically requested no personalization, but that was because I will eventually pass down my collection to my daughter when I'm gone to do with as she likes. If she decides that the collection's not for her, she can sell them. If she decides it IS for her, then she can keep them. There are options down the road. But, when I sent, I also didn't send 13 index cards...
I don't send much ttm any more, since a good number of the Star Wars actors know me and know of Wattographs and I don't want them to mistake a personal request for something that we'd put in the Wattographs store. I'm okay with offering to pay for their signature in order to make it worthwhile for them to participate.
I do like the idea of including a little cash or a gift card if you're asking for something out of the ordinary, but it's also really kewl even if you're doing an ordinary request. Can't hurt to show your gratitude beyond just words.
ALWAYS, however, make it super easy for the celeb to return to you by including return postage and a return envelope suitable to return your items back to you. I've found the easier you make it for them, the better your chances.