It shouldn't really make any difference as the spring just compresses and extends as a function of the load on it and the speed of compression (its only a curly bit of metal trapped between two plates which can move in one direction when said and done). If it were me I'd put the closed ends uppermost for a couple of reasons.
1. With the open coils at the bottom there is less chance of mud etc being trapped in that area (easier to clean) so less chance of rot setting in.
2. If you are fitting lower spring retainer it might be easier to get to (better access)?