J B,
I assume you are talking about making an OIC on the basis of doubt as to collectibilty. From my experience, the best time to make an OIC on this basis is when the taxpayer has little or no assets or income and their future prospects are not very good. If you get a Collection Information Statement from the client, then read and follow the 656 instructions you can make an informed judgement about whether the OIC is viable and has a chance of being accepted. IRS recently came out with some guidance on "pre-qualifying" for OICs. Here's a link.
http://www.irs.gov/Individuals/Offer-in-Compromise-1
OICs can be time consuming. If after an analysis of the client's situation, they can tell you if they want to pursue the OIC.
If they do, make sure you get a retainer up front.
If the client has little or nothing, see if they can borrow from relatives to pay you and fund the OIC too.
Good Luck,
JerryW