I did it with a real nice set of comps before. On my old truck, when I switched out my good stuff and left a set of old speakers I had sitting around in it running off the head unit just to have something to fill the tweeter holes when I traded it. Sounded fine except where I had previously had undistorted volume up to like 28 or 29 out of 35, with this setup things only stayed clear to like 20 or so and they started to distort before they could really come alive. It's not that it will sound like crap, it's just that the higher the clear volume, the more clearly you will hear all the frequencies and the more you appreciate the musical detail. The less clear volume you have, the less you can fully appreciate the sound quality.
Not something I would do long term but if you just want them mounted up and not lying around until you get around to buying an amp, no big deal. Your head unit does not put out nearly enough power that any minor distortion is likely to kill them, unless maybe you put it to the max and keep it there continuously. Too much or too little power does not kill speakers. It's what idiots do with the controls that kill speakers. If you crank it as far as it will go and they're still not good enough, live with it until you get an amp rather than cranking the volume and bass and treble boosts.