According to Apple the Hard drive, and RAM in your computer is "user replaceable" and will not void the warranty on the entire machine. If in the process of changing the hard drive or ram you damage something else then you will not be covered.
If the new HD or RAM fail due to defect, you will have to get that 3rd party to warranty that specific component. A HD will not damage the rest of the computer unless you physically damage something during the swap process.
So dont worry, you putting in a SSD/HD your self will not void the warranty of the machine.
If in doubt just call Apple and ask them, they are obligated to tell you exactly what you can and can't do with your computer.