[lordherald]

I would vote for the mylar covered bubble wrap also. If you compress fiberglass or rockwool you lose R-value and because both are springy (rockwool more so) they will push back against the tape. If you use a product that is as thick or less than the channels in the doors then you eliminate this problem.
Are the panels aluminum, steel or pvc plastic? There is also some doors that are a plastic foam composite.
The metal doors might get condensation on them regularly which will effect the tape's ability to stick.
Fat Charlie's method should eliminate the need for any tape if cut to the correct width.
As to whether you should do it now depends on a number of things. It will help to control the temp in the garage and if it is connected to the house and used as the entryway, can act as an airlock keeping out the heat in summer and the winter cold.
The bubblewrap should be available in widths up to 48" for pole barns and metal shop buildings but you might have to order it.

On a side note this same product makes an excellent sound deadener under carpets and behind door panels and best of all it can be glued into place with spray adhesive like 3M 777.