As to explicitly dropping Temp Tables as soon as they are no longer needed goes, "It Depends". Dropping Temp Tables in a batch oriented stored procedure will probably be fine although it could cause an unwanted recompile. Of course, that recompile could be very short and sweet compared to continuing using the memory that a Temp Table will take (and, yes, they start in memory and spill to disk only if they get too big). For procs that have a very high hit rate, it's usually better to NOT explicitly drop Temp Tables because, as of 2005, the skeleton of the Temp Table can be cached an reused making high hit rate code a bit faster and more efficient.

Apparently, DROP TABLE is not DDL

Dropping a temporary table in a procedure does not count as DDL, and neither does TRUNCATE TABLE, nor UPDATE STATISTICS. None of these things prevent temporary table caching (so it does not matter whether you explicitly drop a temporary table at the end of a procedure or not).