If you already have a 1.8 application that is running fine then there may be reasons to wait before switching (Plugin incompatibility mainly).
If the application is already written in 1.9 then don't bother. 1.9 is such a large improvement (particularly in regards to performance) over 1.8 that there's no sense in building a 1.8 application. Any webhosts that do not yet support 1.9 are definitely going to support it soon since I can't think of anyone building a new application in 1.8 but I can think of half a dozen people building new apps on 1.9.
There are enough differences between the two that maintaining compatibility for both would be a chore for large applications. In particular, you would need to run your unit and functional tests twice (once for each platform).