Hi Jann,
First of all I should say that when You choose version in "Product information" this does not affect on uploaded content at all -- it's just information for users with which version of iClone this content will be compatible (can be loaded).
Real content version defined by iClone version in which content have been saved before uploading. So if You save content in v6 it will be not possible to load it in v5.5. That's why I always do as I told in my first reply.
Yes, currently a lot of users have both v5.5 and v6, but some one still have 5.5 only.
So, by my opinion, if content does not required new v6 features better prepare it (save it) in v5.5.
If You bother that some users will not understand that Your content also compatible with v6, You can always add remark in product description that this content have been successfully tested in iClone 6.
Of course it's my personal opinion and You should make decision Yourself
Vit