Some people here would say "Summer Ears" is the equivalent of an Old Wivez Tale, and others would say it can be proven. If the pedigree gentics are for good ear length, and the ears of the soe you like are just a tad long, I would say chance it, and make sure you use a buck with short ears.
In Nature, there are animals that do show the effects of weather on their outward appearance. Several peole here have experienced the effects of extremely cold rabbits on the new coat growth of white kits-- it causes the fur to come in dark on the tips...first molt, it all goes away. Some rabbits may , when exposed to very warm temps as they are kindled and mature to the weaning stage, may very well possess some genetics that 'turn on' a genetic switch that tells the ears to lengthen a bit while they are growing. Nature is funny--we do not know al her secrets-- who knows-- summer ears may just show that a mini rex is not carrying dwarf genetics-- this is what I was told by a well established MR breeder-- longer ears+ no Dwarf genes-- which reduces/eliminates the chance of peanuts in the next litter...