I would NOT suggest hiring a coordinator. MOST venues that you work with will have an onsite point person who can (and will) assist you with the process. Many properties have a pre-approved vendor list which will help you select vendors because they already know who will work professionally and courteously and not come in and destroy the place. I also checked out all my vendors online on sites like Yelp, etc. I planned a Sonoma winery wedding from Denver in under a year with nothing more than my onsite and the internet :). And it was PERFECT!!
Some sites require that you at least have a day of coordinator, so you may need one anyway depending on the venue. You probably do not need a full service coordinator if you have a lot of time, but someone to help partially plan may be helpful if not required. Additionally Napa has some pretty strict policies on where you can and cannot get married, so if you do not have a venue yet a planner can help you find one and sometimes gain access to places you would not be otherwise able to find yourself.
I agree with Meredith about hiring at least a day-of-coordinator. Reason is that yes, some sites will require a day-of-planner at minimum in Napa (so that the event does not give the venue a bad rep if it goes poorly) and because there are a lot of restrictions and regulations in Napa County in general (for example, Napa has some heavy sound regulations and my own wedding had to end ~10ish. But knowing this ahead of time is good, because we did cocktails and smores afterwards w/o music.)Also because, no matter how much time you have in advance to plan your wedding, you will likely find it stressful to be in more than one place on the actual day of your wedding to oversee logistics, your incoming guests and also your understandably glowy-yet-emotional self! Simply, having a little help could save you a lot of mental stress, at least on the very day of your wedding - which is the day that matters most.
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