Leaving a note is sometimes ok. However it is rude if:
- the person has an office with a door, and you open the door and go in to leave the note. Handle that by leaving the post-it on the door.
- the content of the note should not be seen by others. Handle that by folding up the note so that it won't be read accidentally by passers-by, and not putting super confidential or gossip-worthy details in the note.
- the note is located highly obtrusively. I react with anger when I find items on my chair or post-its on the part of my screen I look at - as opposed to on the bevel. I don't mind a non-adhesive note on my keyboard but some people do. Handle that with great care - one person's HOW DARE YOU INVADE MY SPACE is another person's "oh, I never saw that there."
Make sure you include information in the note that is not in the emails. Do not write the note in advance and set out to deliver it. Go to their office or desk, and if they're not there, leave a note that says:
Sorry I missed you, came to discuss Johnson reports. I need your help to make the deadline. Can we talk at 3?
They need to know why you need an answer and when, and you need to give them an action item. That would be true of emails too but it's all the more true with such a constrained communication device as a post-it.
If you really need an answer, I would risk being rude. Later if the person says it was rude, you can ask what would be better - not angrily, like "what the heck do you expect me to do?" but patiently, like "how can I let you know when I really need you during a busy day?"
As an alternative to leaving a note, consider sending a meeting request. I blogged about someone else's blog about makers schedule and manager's schedule, and I think it's relevant here.