Although putting a car "up on blocks" used to be the recommended storage method, it's not so anymore due to complex suspension systems/parts etc. Porsche also advises against this in a TSB somewhere saying it's not adviseable to "unload the springs/suspension..." or something along those lines.
For only 4-5 months of storage, I'd just follow their recommended guide of over inflating the tires (to the max sidewall pressure of the tire). I've done this for years and have not had any issues of flatspotting. For further precaution, you could also park so each wheel is either on a piece of wood, thick carpet, or high density styrofoam to keep it off an ultra cold concrete slab.
Other (and some more important) things to worry about while storing your car are maintaining the battery's charge, preventing moisture build up - especially in exhaust and gas tank, freezing brake calipers, and keeping mice/rodents out of the cabin, cabin filter, exhaust and air intake.
Here's a nice concise guide w/ suggestions on how to properly store a Boxster (or any vehicle)
Mike Focke's Guide to Storing a Boxster