this is my advice: it's only a quarter or so per chick, so just do it. i opted not to last year with my chicks because of a variety of what i thought were very sensible reasons. heck, they were sensible reasons, but if you're anything like the worrywart i am, you'll vaccinate because, let me tell you, every single time something's wrong with one of my hens, the spectre of marek's looms over me and probably always will.
i had two BRs become progressively blind with some kind of mysterious cataracts (they're still rather impaired, though not as bad as they were) and i was terrified that it was ocular marek's.
i had a BO exhibiting some neurological symptoms and i was convinced that it was neuro- marek's.
a scab on a wattle can send me dashing after hens looking for lesions.
for the piece of mind, 25 cents per bird is money very well spent. the only reason not to vaccinate is the cost and, from my POV, i've more than made up for my pitiful savings in time spent worrying.