Taking someone else's artwork and having is signed is plain rude, no matter what it is, unless you have their permission to do so. They may have shared it on the internet, but that does not make it yours to do with as you wish. If you're going to do it, ask for permission first and if it is refused, heed it.
As for the whole debate about putting manips in front of the subjects; if it's something like a vampire manip or a elf, or something like that, then I'd say, not really an issue. If it's a manip making them do something they haven't done, whether it is innocent or not, it might be a bit freaky for them. The first type is a fantasy, it cannot be real, the second could be seen and believed (depending on the quality of the manip). My advice would be, don't do it, and if it's anything suggestive, really don't do it unless you know for a fact, i.e. the person has stood up and said, in public, they are okay with it. There are some actors/celebrities out there who don't mind, but they are few and far between, and the principle still stands, if it's not yours ask first.
It's the same principle as showing RPS fanfic to the subject of said fanfic. If it's PG and has them playing superhero they're probably going to think it's funny and cute; if it's supposedly real life they're probably think you're a creepy stalker type; and if it's NC17 anything it's likely to make them uncomfortable. So the best thing to do is never show them anything unless it's solicited for a fan competition or something, which will give you strict guidelines.
There is a line, it doesn't take much brain power to figure out where it is and it makes everyone's lives easier if you don't cross it. Embarrassing your idols is not going to endear you to them, in fact it's more likely to have them turning to their security and making sure you never get close ever again. If you like, just think about it as being polite.
Fandom exists in the don't ask don't tell world. These people could start throwing C&Ds around or threatening people with lawsuits and making our lives hard (whether they would win them or not), but they don't, isn't it only courteous for us to give them the same breathing space?