Here is the book ranking so you can see how Kaplan stacks up and why it's not recommended:
Biology: 1. EK Bio + EK 1001 Bio, non-detail oriented 1. BR/TPR Hyperlearning, detail oriented 3. Kaplan
Physics: 1. BR 2. Nova 3. TPR Hyperlearning 4. Kaplan
Verbal: 1. EK Verbal + EK 101 Verbal 2. TPR Hyperlearning 3. BR 4. Kaplan (Avoid if possible)
Organic Chemistry: 1. BR, by far 2. TPR Hyperlearning 3. EK/Kaplan
General Chemistry: 1. BR, by far 2. TPR Hyperlearning 3. EK/Kaplan
Extra Practice Material: 1. TPR Hyperlearning Verbal Workbook + TPR Hyperlearing Science Workbook, good source of practice passages 2. EK 1001 series, helps nail down basics
It's tough to say whether you're wasting your time or not. Kaplan isn't the best resource, so you aren't giving yourself the best possible chance. Yet, it's not like people can't score well studying solely from Kaplan. If you have the money, I suggest you get better books.
Focus on the AAMC tests and mix in Kaplan #1-6 as well. If you only have enough time for 10 tests, take Kaplan #1-5 followed by AAMC #6-10.