I did take an MCAT knowing that I wasn't ready, voided it, retook and got an excellent score, and am now in med school. So let's address a few basic facts.
1. No adcom or med school ever knows that you took and voided, unless the rules have changed since 2003. The void never appears on your record.
2. The void does count as one of your 3 chances, but it is really not a big deal to request a 4th chance. You are not likely to need it anyway.
3. Deciding on the day of the test whether to void is not a good strategy. You have to go in having made the decision, because you will be tempted to think, I spent all this time, now I might as well find out how I did.
You would have to make the final decision at the moment of finishing BS. At that moment, very few people have any kind of realistic perception of how they did, and most people feel awful afterwards. You will certainly be exhausted. Your practice test scores are the best predictor of MCAT score, and with low 20s, if you were my student, I would advise you to wait. It is certainly true that some people do much better than their practice tests, but that is unusual. You might be extraordinary, but you are probably average, just like the rest of us.
Please note that this advice only applies to someone on their first MCAT who knows that they haven't studied much, will have more time and will do a much better job of studying over the summer.
4. Taking and voiding is an excellent experience to prepare you for the real thing. It gave me a healthy respect for the extreme length, the fatigue, and the nervousness. I was just as nervous for the void as I was for the real thing. Also, with the old written test, you had to be prepared to go 8 a.m. to sometimes 6 p.m. with all the instructions and stuff. Knowing this in advance was very helpful. I had the same experience with the Step 1 practice test (end of second year of med school) compared to the real thing; the practice is invaluable, especially if you already spent the $$.
5. Taking the MCAT is a whole different experience from a Kaplan or TPR test. You will get a much more realistic sense of the question style and how topics are covered. Kaplan simply does not do a good job of emulating the real MCAT style, in my opinion. Again, if you already spent the $$, a very valuable experience.
Good luck!