Must a player disclose what deck they are using before play?
Mostly no, but it depends.

First, here's a relevant quote from the tournament rules (section 2.7):


  Generally, decklists are not public information and are not shared
  with other players during a tournament. At constructed-format,
  Professional REL tournaments (Pro Tour, World Magic Cup, Magic: The
  Gathering World Championship, and Grand Prix), copies of opponents’
  decklists will be provided to players in the single-elimination
  playoffs.


Now for some personal experience: In draft events I have played, it is not normal (although it happens occasionally) to give information about your deck before playing. Many times someone will finish the first game (best of 3) without revealing one of their win conditions, and will keep it a secret until it actually comes out in play in the 2nd or 3rd game of the match.

Thirdly, some reasoning: In casual play I think it makes even more sense to change and keep a secret the content of your deck. I play more Android: Netrunner than Magic, so this reasoning draws on my experience there as well, but in general my goal in constructing a deck is to make one that will be resilient to surprises, and have a chance at winning against a variety of deck strategies. To that end, knowing the contents of my opponent's deck is a hindrance. I would rather we constantly surprise each other to try and polish off the rough edges of each other's decks.

That said, this way of thinking works very well among confident, competitive people, but might be harder to swallow for somebody still learning the mechanics of the game. Especially at the beginning, you might choose to help somebody along by explaining the changes you're making ahead of time, and why you think they'll make your deck better. Well, we've taken a turn into the highly subjective world of interpersonal relationships, so I'd better sign off before I get myself in trouble.