It's personal preference and it depends on what your plan is for redirecting old URLs to new.
If you retain imported content IDs then a thread with ID 100 in vBulletin will have the same ID in XenForo. If you don't do that, then the thread with ID 100 in vBulletin could be imported as thread with ID 90 in XenForo.
For the forum, if you're using something like the redirection scripts, then it doesn't matter too much because they handle the mapping from an old ID to a new ID.
If you didn't have a standard URL structure in vBulletin, e.g. because of using something like vBSEO then it might be that you have to write your own custom redirect rules which might be easier if you retain the content IDs.
It's a similar thing for the Gallery. A picture in PhotoPost with ID 100 could be imported as something different into XenForo Media Gallery.
There are currently no redirection scripts available for redirecting PhotoPost Pro URLs, so retaining the content IDs may be more important here as it would make writing custom redirects easier.