In my experience, you have to consider Days on Market (DOM), and you need to run your own comps. You can use Zillow to do the past sold $ amounts (although that will not have any concessions data, for that you need a realtor to help you with the MLS data). The list price is just a data point in the process, you can't set your price only by % of the list price. In many cases list price is set arbitrarily based on the seller's emotional state, or set too high by a realtor who was hungry for the listing, knowing they'd reduce it later. Do NOT take the word of the seller agent for comps. Even having a buyer's agent is not insurance against poorly priced properties - your agent only gets paid when you buy, so they will do everything they can to make a deal happen, not necessarily save you money.
What you'll find is that some properties are listed way outside the range of similar comps. When we looked in our area, we found a few good properties that were 20% or more overpriced. We had zero luck explaining this to sellers - eventually we gave up on badly priced properties and simply stopped looking at them. Do this before you even look at a property.
Once you find a home that's priced correctly, you have to look at DOM. I did some data analysis of all sold properties in my area for the year prior to us buying, and found that properties that are under 60 DOM sold at 95% or higher of list, or they didn't sell at all. Sellers are much less likely to consider lower offers in the first two months. The analysis I did showed anything over 60 days sold for between 80%-95% of list, with the average over 60 days selling for 90.6% of list price. Median was 90.7%.
So once a property hits 60 days, you can loft some 85% of list offers out there. But you must run your own comps and only bid on stuff that's not ridiculously overpriced, because sellers get emotionally attached to the number they've been sold. You might get a lot of sellers coming back with rejections of your bid. We had a seller reject a bid that was 91% of list (it was listed high, we bid 102% of the actual retail value, they rejected it). It will be frustrating, but just walk away. There is always another house to bid on.
I've got some stories about our real estate misadventures in my journal:
http://forum.mrmoneymustache.com/journals/cleaving-to-fi/msg953225/#msg953225
