Idols- to tweet or not tweet to?

Ok, something has been bugging me for quite a while.

I'm pretty new to the Twitterverse, so my hash tagging and my quotes are sometimes weird. I also live in a part of the world that's one of the last places to get news because of our time zone. When idols tweet at night, I don't see them in my feed until I wake up the next day. Hours later.

I follow a good part of the South Korean idols, actors, etc. that I like. And more of course. It's cool and exciting to see what they're up to. And the selcas (self camera shots) are funny sometimes, too.

My questions are: when is it ok to tweet to an idol? Like an "out of the blue when you feel like it" tweet? Do you only follow them and reply to their tweets? Is it ok to tweet stars like Lee Hong Ki, Lee Min Ho, CNBlue, Wonder Girls and more questions like "how are you?" or "what have you been up to?" as long as it's not too personal? And when is it too late to reply to a tweet? I feel embarrassed replying to anything more than a couple of hours old. It feels like I'm replying to no one because everyone else has said their peace and moved on.

I wouldn't want to go crazy like sasaengs, wanting to know everything about stars...But is it too much to do that? Social media sites like Facebook and Twitter open up communication. But is there some sort of unstated protocol?

Wildly curious...

And I definitely wouldn't want to offend...