There's a few variables to consider depending on where you are in the evolution of your business (hunting, retention, up-sell), what the product is, price model and how much "touch" or knowledge transfer is involved. If there is a good deal of training, integration and interaction, I would get the AMs involved during the sales process and closer to the decision point. If an AM has a high client load (and low interaction), I think you are safe to get them involved after the signature.
If your hunters are getting to the point where an AM is needed, you need to set expectations with the AE about what happens to the client when they sign. They should sit with the AM to do a knowledge transfer and schedule a transition call with the client and the new AM. There may be some exceptions where you have some AEs who are a good fit with the client and it may make sense to keep them on the client. That may be an indication you have a good AM or farmer on your hands.
Depending where you are in your evolution of the the business, client acquisition, retention, up-sell/cross-sell - if you're at an early stage of the business - hunters will be sufficient. When your hunters aren't spending much time with the clients (except for renewal time) or spending so much time with clients that they don't have much time to hunt, you should develop an AM team. Offer the AEs a chance to apply for an AM role. If you have some key accounts that are large and complex, you may need to look at having an account team which includes an AM and farmers. Setting out why you are doing this, expressing expectations and minimizing exceptions are key.