I am having the same issue and found this link that is related, but not helpful to me. It states that the semantic query for paramters is not sorted, so you could try indexing the source by the field. I think that this is a needed enhancement by Microsoft - to sort paramter drop-downs automatically (probably by the sort options that are already set for fields in the model - hmm - what a concept!)
This may help you, but not me because I am using a view that resides in a separate db from the db that is being accessed and to index a view it apparently needs to be schema bound, which I cannot do because it is another db and because a vendor may change the table in patch updates that I don't want to mess up by binding.
Any new thoughts would be nice.. Or if somebody can twist microsoft's arm to add this enhancement to sort parameters in report builder...!
http://technet.microsoft.com/en-us/library/ms160347(SQL.100).aspx