Without a Crystal Ball to look into, I can only go by what I see reported through the local MLS.
In the first quarter of 2012, we saw 232 single family homes sell Keys-wide. When we look at the list price to sell price ratio, the average is 92% of asking price. The numbers are slightly different for condos; 110 properties sold with a list to sale ratio of 95%.
So, given those numbers, should you automatically based your offer 5 to 10% below asking? Probably not.
There are a lot of variables to consider, such as condition of the property, location of the property, and the length time the property has been on the market, to name a few. Properties that have easy or immediate access to the water tend to sell for closer to the asking price. Properties that have been on the market might have owners that are now willing to accept less than what they are asking.
One option you have is to ask your Realtor to provide a CMA (Comparable Market Analysis) for the property you are interested in. Properties used in the CMA should be as closely matched to the subject home, much as they would be in an appraisal of the home. You need to be sure that both on market homes and sold properties are included in the analysis. There should be as few adjustments as possible done by the Realtor because most Realtors are not licensed appraisers and should not evaluate property the same way appraisers do.
You should also know the market activity in the area you're interested in. If the market is a buyers market, with several homes competing against each other, sellers may be willing to accept less to avoid chasing the market down. But if sales are on the rise and the inventory is moving like in a sellers market, sellers are less likely to negotiate on their asking price.
Have you seen a property you'd like to laern more about? Feel free to ask me for a no cost, no oblligation CMA. I'd love to help you with your purchase of a home in the Florida Keys.