Allison Shields has a really good post on shifting from hourly to fixed pricing. Think about how her paragraph, below, illustrates a way in which hourly billing has become a serious "safety net" -- but at the expense (literally and figuratively) of the client! And not without negative impact on your client relationships, too.
"Often lawyers [sub: accountants] take on a case without fully investigating or exploring not only the facts surrounding the matter, but the client's specific expectations and values. One advantage of changing the pricing structure is that these longer and more detailed conversations will likely help lawyers pre-qualify clients better, alert them to potential problems earlier, and help them weed out ‘nightmare’ clients before they become a headache."
Skipping these important conversations and the subsequent thinking about project depth and scope is only made possible because hourly billers know they usually won't "lose" no matter what since they'll bill what it actually "takes."
This lack of communication on the front side results in our lack of understanding/planning and the high likelihood of our client being "surprised" with an unknown bill amount (usually higher than what we or they would have anticipated) is an absolute service faux pas.
Of course this shift of all risk to the client (from us) means the client loses in every way because they go into the project even more blind than us. Some clients will certainly object to the final price, but with hourly billings, you won't know until AFTER you've done the work--when you're on the defensive and they've received the benefits of your service. Some "nightmare" clients wouldn't be nightmare clients at all had they been given the opportunity to make a more informed purchase in the first place.
Allison also makes some strong points about setting up more complex projects in phases or stages and determining/articulating the value of the work you are doing as opposed to our typical default conversation about "how long it takes."
Without discussing project scope/intensity and value up front, you cannot possibly set expectations. If you cannot set them, you sure as heck are going to have a hard time meeting them!
Think about this:
With many legal and accounting services, the client is facing unknown processes (or at least processes outside of their comfort zone) with unknown--an largely unguarantee-able--outcomes.
Shouldn't the client at least be able to count on a pretty well-defined price (at least phase by phase) and clear communications from the professional, in advance, about the same?
Hourly billing's 'safety net' has encouraged professionals to withhold the few assurances you can actually provide to them.