Elvis has the perfect answer. You advise your clients...provide the best advice possible...then, as an agent, you do what your client wants.
Two other observations: First, I wouldn't consider 10% under the list price to be a "lowball," unless the list price was already substantially under the comps. Consider: Most sellers expect some negotiating to occur. Even in today's market (read some of the other posts on Trulia) many sellers will leave a cushion to negotiate with. So, you offer 10% under list, they counter at 3% under list, and you agree at 5% under list. There you are. Consider, also, that in many markets, the average home sale is 4%-8% under list. So--unless the house is a clear bargain at list price--I wouldn't consider 10% off to be "lowballing."
Second point: I come from an investor background, where offers of 20%-30% under market aren't unusual. And investors don't hesitate at all with those figures. But the difference is that the investor mindset is: "The numbers have to work for me. I know most of my offers will be rejected. But I'm pretty confident that if my numbers are good and an offer is accepted, I'll make money." The mindset of many Realtors is: "I really want my client's offer to be accepted. And I'm not going to get paid unless the offer is accepted. So, I want the deal to work, so everyone will be happy." And there's nothing wrong with that; most Realtors really do have their client's interests as their top priority. But, sometimes, the client's real top priority...what would be in their best interest...is to get the property at 15% off list, or else move on to another property. Some agents have difficulty appreciating this.
Hope that helps.