I wouldn't call using una corda to play softly cheating at all. However, una corda is about much more than volume. It is most effective when used to change the quality of the sound. The key to any piece, soft or loud, is not "playing really soft," but rather establishing from the beginning a dynamic range that is wide enough to hold interest. For effective color change, use una corda.
To begin any piece with una corda (unless aspecific effect is called for) is generally not a good idea, as the timbre is less inviting to the listener. Consider, too, whether you will play the etude separately or as part of the whole opus. Especially if the latter, do you really want to start out with such a thin tone? I don't think it's good for the audience.