I would just disconnect the+ side, then stick a charger on it a week or so before you plan to use it. I don't trust an unattended battery tender. I have had that little controlled box get hot and swell up on a few.
On a negative ground system, it's best to take the - cable off. I was taught in A&P school, I think it was, to always take the ground off first and hook it up last. If the + cable is on, your wrench or other metal object will become a ground and cause a direct short, just like you had touched both - and + at the same time with a conductor.
