It depends on the agent.
A good agent with a good rep will value things at what they think it is worth and expect to get that.
A rubbish/struggling agent will over value to appeal to the vendors greed so that they give them the property to sell. A lot of the time the house is on the market longer and either drops to the original valuation price or the vendors go with another agent.
In agency the biggest thing is getting the houses on, not selling them! Selling them is the easy bit.
Plus think about the percentages, on that 230,000 house if the seller is on 1.5% commission he'll have to pay the agent £3450 round about. Now if e sells at £210,000 the agent gets 3150. Now for the sake of £300 is the agent going to care that it's 20k under the asking price when all he loses out on is £300? No he/she isn't

So don't always think the agent is there to get you the best price possible, the agent will sell the house at a price he has to.
That said, sometimes the overvaluing pays off for them, esp in the recent market, and they sell for the higher prices.
Personally as Rotty says, it depends on how similar stuff is selling in the area. If it's going like hot cakes then the vendor will know and will be expecting near enough the asking price, I'd be putting in a 'cheeky'' offer of £220k and working up from there. If the market isn't so hot there then I'd offer £215k (and expect it to be laughed at, but you never know

).
When buying our place the vendor said pretty much outright what his bottom line was, we tried offering below but didn't get anywhere. In the end for the sake of the couple of quid we said ok then, that was 1.8% under the asking.