In my market sellers generally price their homes about 5% above market value, so any bid coming in at 95% of asking price is very acceptable.
Beyond that, it is truly an individual decision. When I'm representing my buyers I research the property to get a rough idea of how much is likely owed on the property, and then see what other information the listing agent is willing to provide (what is the seller's motivation, are they able to provide financing, etc.). This leads me to a likely sale price.
I can also tell you that in my market an offer 20% below asking price is likely to be totally ignored. What's happening in your area may be entirely different.