Depends on the employer. Most should let you opt out (though you may want to check how that could effect your taxes with current laws [ie "Obamacare"]). IF you have an dependents, you CANNOT opt out if you want them covered. The only way to cover them through an employer is to cover yourself, too. There are some good companies out there that cover the employee for free...all you'd have to pay for is your dependents.
I've HEARD (don't know...hopefully someone can verify/clarify) that if you're insured, the VA will reimburse the deductible and any other expenses...and bill your insurance to help pay for your fees when you go to them to help save costs. Again, what I've heard...not a fact.