Has there ever been any previous discussion about being able to select those?
My suggestion is to at least enable the 'defender' to choose their default posture when attacked. For example, they might be an aggressive fighter or built around the concept of having those slight offensive buffs from the more aggressive postures. Why not let them have a "hair trigger" and violently respond to their attackers?
I think that allowing the 'attacker' to choose a posture could lead to some problems. For example in this situations where people are built to be evasive and benefit from a guarded stance. Allowing them to initiate a fight with a strong defense seems like a potential balance issue.
At the risk of throwing 'realism' into the mix here, I'll mention that it isn't realistic to start a fight defensively. Two combatants in defensive postures aren't actually fighting. Attacking someone requires leaving yourself open, however briefly.
On the other hand, it's totally realistic to come across people who believe that they can take a couple of hits and who are willing to trust their stamina and training to make up for a lack of defense.