Short answer: Yes
The short answer is yes, you must have an account manager field on one of your client forms. Choosing the account manager lets Senta know who should receive most Tasks relating to the client.
Long answer: Probably
The longer answer is still probably yes, but some practices don't have account managers, in which case adding one will do no good!
As you know, Tasks are currently allocated to the account manager if the Task is set to Account Manager. Whenever Senta finds a Task to do, and discovers that it's set to Account Manager, it looks up the client and finds out who the account manager is. Then, it assigns the Task to that person. If the account manager for the client changes in the future, Senta will reassign the Task to the new account manager.
However, there are other ways of assigning work through Senta. For example, you can assign certain Tasks in a Job to specific people. For example, if Alys always does client take-on Tasks and Ben always does accounts production work, then you might ask well set those Tasks in a Job so that they're assigned to Alys and Bob.
You can also allocate work to "specialists", which are like account managers but who do a particular Task for the clients, e.g. accounts or tax work.
If no Tasks in your Jobs are set to be assigned to Account Manager, then you don't need the account manager field and can remove it.