Should I retension my tapes before use ?

I don't think so. Here is why: The biggest reason is that it reduces the life of the head assembly, which is the most expensive part in the machine. Very few head designs will float the tape over the head, only those without a space or gap between the upper and lower cylinder will do this. These head designs will have a "cup" shape around the actual head. Also, I have used thousands of tapes without retensioning and NEVER had a problem. Audio grade tape in studio use, with heavy jog wheel and search operations, do not require retensioning the tape. The normal play thru and rewinding of these tapes eliminate the need for retensioning. This does make sense with DDS tapes in DDS applications (this is where the idea came from, not Audio Pros, although they will repack a master tape that has been handled improperly before transfering data) as the tape is started and stopped quite a bit and no transport handles this perfectly. So with time a DDS tape may require retensioning in order to perform correctly. Please note, if you use 90 meter or longer DDS tape, you should retension your tapes periodicaly. This will minimize data loss and tape damage.

Bear in mind that audio grade drives are not designed to handle 90 meter and longer tape and their use does cause problems.