It's not optional. If you are providing more than half your own support, you parents are not allowed to claim you. See dependency rules below.
If you live with them and are not paying them rent, it's unlikely that you are supporting yourself. IRS Publication 501 on page
15 has a worksheet that can be used to help with the support calculation. See: http://www.irs.gov/pub/irs-pdf/p501.pdf
The support value of a home is the fair market rental value, divided by the
number of occupants.
The "status" you claim at work (form W-4) is not binding on how you actually file a tax return, so you can change that mid year. It's only an estimate. But, there is no half year dependency allowed on an actual tax return.
Usually, not being a dependent is reason enough to be disqualified from parent's health insurance, but you'll have to check their rules for a definitive answer.
If it was optional, the only way to know which is best (claim yourself or have parent claim you) is for both of you to prepare returns both ways and compare. But the general rule is it's better for the parent's to claim the student.
