I can't think of a single medieval battle where two commanders made some kind of agreement to fight a full-scale battle in the field at a particular location. At Hastings, William didn't have a great deal of choice. At some point, if he was to actually conquer England, he would have had to eliminate Harold. Harold, for his part, couldn't afford to let the Normans establish an uncontested foothold, and so marched his army south to deal with them. Once in the area, he knew that the Normans would have to come to him, so he could establish a defensive position on a hilltop.

Remember, armies are huge, expensive entities that take vast sums to field and maintain. Large-scale pitched battles were relatively rare. Many field engagements occurred as a result of sieges: one side would attempt to take a town and the town would send out messengers to bring back reinforcements. If the reinforcements came in time, the enemy might break off the siege and attempt to either flee or attack the relief force in the field. Typically, there would be a period of maneuvering as each army attempted to surprise the enemy or simply discover where the enemy was. This was a time before modern communications technology, so the typical method of scouting was to send out small patrols. These patrols would run into each other and clash in skirmishes before the main battle. When the opposing armies figured out where they were in relation to each other, the commanders would try to pick the best terrain to fight the battle on. Being on the defensive could be a major factor, because it allowed that army to establish their formations and fortify their positions. Skilled defensive positioning enabled most of the major English victories of the Hundred Years War.