It is fine if you don't send them! A lot of creators don't make them, you wouldn't be considered rude! ;u;
If you do a search on the Tapastic forums for "thank you" you'll probably find 2 or 3 other threads with people discussing how they feel about them -- when I read through, the majority of people like getting them, so I just have a note on my profile saying to PM me if you don't want one, and I always check the person's profile first to make sure they don't say to "PM me instead" or something like that before I post on their wall
The other thing I would recommend is to make sure your image isn't suuuuuuper tall! Tall images take up more room on a wall than wide or squareish ones, so that's kind of a courtesy thing, just making sure you're not taking up a whole ton of wall-space. ;u;
I actually try to wait through at least one update in case someone changes their mind -- I want to give them time to unsubscribe b/c I don't want to accidentally make someone feel bad for unsubbing. xD This..... is part of why I'm perpetually behind in thank-yous, though.