I work and live in Scandinavia, and by culture we use a person's first name when speaking to someone, colleagues, clients and so on. In general we use the second-person singular when addressing someone, we are very informal and being formal can be seen as if someone is a bit of a stuck-up. I have been told on more than one occation that I am very polite.
I work in an international setting in design engineering, with my supervisor being German and contact with clients who are both European and American. I have the habit of always addressing my clients in mails and so on with their title, if I know it, or with Mr/Mrs/Ms etc. and their last name.
When in contact with clients they sometimes reply with my first name in mail or telephone conversations, whereas I stick with Mr and so on.
Would it be considered rude not to say their first name after this kind of an invite? I have nothing against it, but it feels somewhat forced to have a jovial conversation calling eachother Mike and Eddy one day and the next time they contact me they can be completely livid because the product does'nt work as they expect. They usually don't call me by my first name at these times...