Sure. When your watercolor painting dries, it's perfect as-is, and you need do nothing more, especially if you know it's going in a drawer or a frame or even in a pile of other watercolors in the corner, lol. However, it will react with any kind of moisture, because that's what watercolors do. If your watercolor work gets wet or is in a really high humidity area, touching it or allowing it to touch anything else will smear the paint.

So if you have some work you really like and want to protect when it's done, you can use a surface fixative (usually a spray) to protect it from moisture. But greeting cards don't usually see a lot of water or humidity, so it's up to you. It will not be obvious either way: fixatives are completely invisible to the eye until water is involved and then it's pretty obvious what's been sealed and what hasn't been, lol.

I have a couple watercolor paintings hanging in my bathroom, for example, but it's not a problem because I sealed them with a surface fixative as well as Dorland's (a wax made specifically for artwork) and they're behind glass, so the occasionally high humidity in there doesn't matter at all. But that's kind of an extreme case.

Fixatives are pretty cheap and a little goes a long way, but it's up to you as to whether you want to use one. If you do, just remember that the fixative is the very LAST thing you put on, because you usually can't work the painting after you've done it. (There are "workable" fixatives that you can put on and then still add to your work, but just how "workable" they are really depends on the specific fixative and the specific media you're using, so don't count on it.)

You said you are making cards; IMO you would be fine leaving your work as-is or sealing them. Honestly either way is fine, your choice. But if you do use one, make the fixative the very LAST thing you do to your art, at least until you have a chance to experiment with any fixative you buy. And of course, do any ironing prior. Hope this helps.