The battery should be about half-charged - that's better for long-term storage. So you should be able to just turn it on straight away - I did with mine.
Just turn On. In manual they specially answer on this question in FAQs. It is sound like this: N900 used Lithium battery. For long storage better use 90% of charge. And It could be used out of box without first full charging....
Plug it in (because you can't be really sure what the battery level is), but after that you can go ahead and power it on if you're impatient. This is basically what it says in the manual.
