should you offer 5,10,or 20 % less than asking price

First of all, not all real estate agents are "blood-suckin'."

As far as what to offer, you need to consider many issues: 
What is the current market value?
Are there other offers?
How long has the property been on the market?
Are the sellers negotiable?
Is it a short sale or foreclosure?

If the list price is completely off base as far as market value, then you also have to consider that you may be dealing with an unrealistic seller who believes their home is worth more than its current market value.

If there are multiple offers, then you may have to offer at or above market value, or move on to another home.

There is no set percentage that any honest real estate agent can recommend you offer below list price.  What if the home is listed below market value for a quick sale?  What if it is overpriced?  What if there are other offers?  In many areas, the final sales price is between 92%-97% of list price, but these are just averages.

You need to work with your real estate agent to determine market value, then create an offer that will be a good starting point for negotiations.  Of course, you will need to learn to trust and work cooperatively with your agent.

No!

You should completely forget "asking price"; and offer between 40% and 130% of "market value", depending on other conditions and what the market value will be in 3 years.  Or completely walk away and forget the property in the first place.