I am sure someone with more experience can help better than I but I will take a shot at it. When tillering for myself I will draw until one of three things happen:
1. I hit my target weight.
2. I hit my target draw length (hopefully at my target weight).
3. The most common durring the tillering process, I see a flaw. Either a flat spot, hinge, or one limb bending more than the other.
If you are drawing and not noticing any problem areas try taking a picture of the tiller. It will allow you to take a closer look without leaving the bow under stress.
There is also a tool you can make out of a block of wood (I haven't made one yet and the name escapes me) but you can use it to run on the belly of the bow to help point out flat spots... Please someone help me out here with this one.
All of my bows so far have taken some set but not enough that I notice it on the tillering tree as I go. For me I notice it after I get to full draw and shoot it a few times. I try to go just over my target weight off the tree, so after shooting in and figuring a bit of set and loss of draw weight.
I hope some of this makes sense and that my method is not way off base.