Is it preferable to wait for Version 1.0 instead of implementing a beta?

This very much depends on the flexibility of your IT situation. If it is quite flexible and can easily be expanded with new fields, it is worth using openfunds from an early stage, not least because you are probably already filling spreadsheets built on the openfunds standard.

If implementing new fields causes difficulties, it would be better to wait for Version 1.0 as the release cycles of new openfunds versions will be considerably longer after that.