Yes, anyone who analyzes the messages you send can assume that you are using SMSSecure.
However, compared to the amount of metadata that's already being leaked over SMS[1], adding the fact that you could[2] be using a specific SMS client that has the ability to encrypt messages doesn't seem too bad.
There was an option in a previous version of TextSecure to disable this tagging, but it was deemed unused and axed[3]. For the same reason, I'm loathe to add it back in, but having the option shoved under the "Advanced" menu may not be too bad.
[1] This is something that TextSecure does much better with. SMS messages (even encrypted) still leak metadata on who you're messaging and when.
[2] There's some element of deniability with whitespace tags (granted, not a lot). On the other hand, if you're registered with TextSecure (which can be checked simply by adding a user your contacts and opening the app), there's only one reason you would be there.
However, compared to the amount of metadata that's already being leaked over SMS[1], adding the fact that you could[2] be using a specific SMS client that has the ability to encrypt messages doesn't seem too bad.
There was an option in a previous version of TextSecure to disable this tagging, but it was deemed unused and axed[3]. For the same reason, I'm loathe to add it back in, but having the option shoved under the "Advanced" menu may not be too bad.
[1] This is something that TextSecure does much better with. SMS messages (even encrypted) still leak metadata on who you're messaging and when.
[2] There's some element of deniability with whitespace tags (granted, not a lot). On the other hand, if you're registered with TextSecure (which can be checked simply by adding a user your contacts and opening the app), there's only one reason you would be there.
[3] See https://github.com/WhisperSystems/TextSecure/commit/40eca5e0...