In most situations I prefer to hold my cav units. Particularly in a defensive battle I tend to send one or two units out wide early. My infantry take the brunt of whatever my foes have to deal out, and then my cav units charge from the rear and/or run down routers. Any extra units that I have held behind my main force also go wide and hit the enemy's flanks at the same time that the original flankers hit their rear.

If I have mounted archers of any description (generally Turcopoles as I have yet to play a muslim faction), they also go wide early, with their mission generally being to find a spot from which to pepper the enemy general.

On attack I follow a broadly similar approach....

In bridge situations, if there are two, I will often send some cav units to charge across the undefended bridge (why does the AI leave the back door open so often?) purely to get the AI to split their forces. On a one bridge battle, I hold them all until it is time to slaughter routers.

I suppose that because of this approach, my cav suffers less than other units, but they still can take a few hits from time to time....