It will not be necessary to reserve something in advance, especially in off-seaon. I was in Putuoshan for two nights about February 20, 2006. When we got off of the boat, right inside the ferry terminal building there is a desk with some people with telephones and computers to book hotels on arrival. However, I didn't like the idea of paying rack plus a 15% commission. So I walked out of the building to a helpful English-speaking "hotel hawker" with a van and cell phone. I asked him how much for the Baotuo Hotel, which seemed nice from the Lonely Planet or Let's Go books. Paid 250 yuan (about 100 yuan off high-season rate) per night for my wife and me. I would say it was one of the better values, because other places ranged up to twice as much rack rate and did not seem any better or were clearly worse.

The Bautuo Hotel has a western-style facilities, is clean, and the hot water gave out only one of the two morning. Its attached restaurant was better than most of Putuoshan's mediocre offerings (and that's faint praise, indeed).
The hotel is well located to the sites.

Of course, there are other options, as you mention, that would be much cheaper, but I couldn't speak to them.

Bowgie