The current version is 6.0. There are nevertheless numerous variants, mainly in the colour depth, in the compression scheme, etc. This means that just knowing you will have to use a Tiff file from a third party can be both a reason to worry and a reason to keep cool. It happens too often that you can't mount a Tiff file in a DTP package and only get a cryptic message saying that this kind of Tiff can't be opened.
Experience shows that surprises are less frequent with professional graphic packages.
If your budget is tight, check that the files your are producing can be opened in the major packages, mainly if you have to go cross-platform. However, we would like to stress that it is better not to save in a compressed variant when you have to give a file to a customer who will use it on another platform. In the same vein, DTP users should insist on getting uncompressed Tiff files from their graphists. If you want to save storage, save it on your archive media, not on the Zip cartridge or on the CD-ROM you give to your customers. Well, things are never so simple.