Email greetings at university

A month or so ago I came across this blog and post:
FemaleScienceProfessor: How To Annoy Your Professor

It was a poll on the things that professors find annoying in their students. There are quite a few and I can definitely see that some of the would be annoying. There was however one that made me wonder...

I'm talking about email greetings at university.

According to the blog informal email greetings are inappropriate and annoying. I've been starting emails to quite a few of my lecturers with hello over the past few years and to be perfectly honest I have never  thought that it might be inappropriate. I'm slightly... Well, worried is a big word, but I certainly don't like the idea that I might have been accidentally rude to people I respect.

I use appropriate titles whenever I write to a lecturer I don't know very well, someone I've never written to before or if it is only a one-off question or administrative/technical issue etc. However, when it comes to people I correspond and work with on a regular basis I tend to use whatever format of an email the lecturer uses. For example if the lecturer signs their email with first name alone, I'm assuming it's alright to use their first name in the reply, even if they have never directly said I can use their first name. Similarly, if their emails start with plain hi is it  really rude for me to reply with hello?

Is hello considered rude in general? Or are there differences between countries and fields? Or maybe it just depends on the relationship between the student and lecturer (doctor/professor)? What I gather from the blog is that the lecturers over the pond would like to be called professors whether they have a professor title or not*. I partially understand - my high school teachers were called professors too (even though none of them had a title higher than Masters), it's a cultural thing. But is it the same at the UK universities? How important are the titles in such correspondence?

I feel like this should be one of the things taught to first years, University 101, as students, especially international, might simply not realise what the rules are. I have been in the UK for a while and clearly I still don't know some very basic things.

*I'm deliberately not going into the issue of men being addressed as professors and women being addressed as Ms/Mrs.