We just got back from a 5 night stay at BCV. Checked in before 10am so we could use (and wanted) tickets for that day but not for check-out day. I did not mention dropping days at all, and neither did the CM. He just brought out a placard describing the UPH pass, and said he could sell me tickets for either 4, 5, or 6 days, and which length would I like.
As it was presented to me it seemed like policy, at not at all like CM discretion. The key here is that it was not presented at all as "dropping days", but rather as my choice for number of days.
By the way, the UPH is the only pass available at check-in with the DVC discount. Plain park hoppers with advance purchase are a little cheaper than UPH with the DVC discount. For example:
5 days UPH- 277.97 - 10percent is 250.17.
5 days park hopper advance purchase is 231.11.