It is a clear violation of Starfleet safety protocols and also practically impossible to do in the middle of the battle. Also, when object goes FTL it becomes heavier in the scientific point of view and surely would damage the ramming ship beyond acceptable levels. Besides, Dominion ships used impulse-ramming as a very effective way to disable enemy vessels.Because if the other ship moved even a tiny percent, it would be difficult to impossible to correct your heading in time to ensure collision. You would need a very large target that wasn't moving, and to begin your run very close to the target. Riker was about to try it with the Borg ship in The Best of Both Worlds Part II.
Janeway: "Tom, what's the first thing they teach you about maneuvering at warp?"
Tom: "Faster than light, no left or right. When possible, maintain a linear trajectory. Course corrections could fracture the hull."
Not to mention the fact the ramming is a bit a last ditch effort which would generally result in significant damage if not destruction of both vessels. So if the commander is considering using such a tactic the ship is likely going to be very damaged and probably would not have warp capability. If not and the ship did in fact have ftl travel systems online you'd probably want to fly away from the battle not into the middle of it.
Well, think of it this way, by the time you're willing to ram your ship you will be about to get blown up. How can you even get a stable warp field, if your warp core is still online?
As well as that, ramming at warp would completely destroy your ship, plus, as said before, if you miss then the hull fractures you would have from the battle would become hull breaches and emergency force fields need power so...