I'm not sure entirely what kind of disclaimer you have in mind, but the point is typically more to protect you as a developer than to guarantee something for the users.
You basically want to explicitly state somewhere that you're providing the software "as is" and that you should not be held liable if the user misuses it or if the software malfunctions and damages the user's machine or data.
You mentioned that you're using WTFPL, and it suggests the following wording:
/* This program is free software. It comes without any warranty, to
* the extent permitted by applicable law. You can redistribute it
* and/or modify it under the terms of the Do What The Fuck You Want
* To Public License, Version 2, as published by Sam Hocevar. See
* http://sam.zoy.org/wtfpl/COPYING for more details. */
With this notice in place, you're not offering any implied warranty.
Do you have to add a notice like this? That depends entirely on where you live, what your laws are, etc. I'm not a lawyer, but I don't see a situation where putting a notice like this in place could hurt you, so I'd add one even for free software. If you think that the app you wrote is worth the hassle, you can also consult a lawyer to make sure you as a developer are fully protected.