Both movies are different takes on the show that are not in the same continuity as the TV series (or each other), so you could technically watch them at any point.
That said, the first movie (House of Dark Shadows) is kind of a remake of Barnabas's arrival at Collinwood - the plot is different, but there's enough similarities that I'd recommend not watching the movie until you've finished the 1795 flashback, so roughly where you are now.
Also, the first movie used the cast from the tail end of the TV series, so although there aren't really any spoilers it might be more satisfying to watch the movie right at the end of the show so you can pick out all the familiar faces that pop up.
The second movie (Night of Dark Shadows) is, as far as I recall, a completely original story in a completely different continuity to the TV show, but uses several actors as new characters. It revolves around Angelique and Quentin, and their personalities are broadly similar to the TV characters, but they're basically 'parallel universe' versions - so as long as you're familiar with them the film won't spoil anything for you.
As to whether or not you *should* watch them - since they're not part of the TV series story, you could skip them if you like. I'd recommend them though - they're not the best movies in the world, but they're pretty fun, and it's interesting to see different takes on the show from the original cast and crew.
