There is a tiny risk that a rattler will cozy up to you in the middle of the night for warmth. The slightly larger risk is selecting a site by their den exit/entrance. But I would bet that it's more likely that you will be hit by a cross town bus.

Do you need to keep closed off shelter to keep snakes at bay? In the US... I don't think so. In other locations in the world... sometimes.

I would suggest the tent -vs- tarp is mostly a personal preference thing. If you are the sort of person that will worry about snakes even though there is little risk... then take a tent and get a good night of rest. If finding data that the snake risk is small calms your heart, consider a tarp.

Of course the are the compromise of tarptents that are almost as light at a tarp but still provide full bug/snake protection.

--Mark