Let's face it, the knights already cost 23 pts, you have invested quite heavily so there's no point trying to scrimp and save on what might become your most combat effective unit.
The upgrade gives you a str of 4 rather than 3 and costs 3 pts. It's worth upgrading a normal knight to a preceptor too, his strength upgrade to Inner Circle knight is also only 3 pts.
If you take a large unit of knights with a decent rank bonus of 2 or more and a standard. Then don't waste your time going for standard rank and file enemy units with a toughness of 3. The strength bonus has no effect in the first round (if you charge) because you need a 2 to wound whether or not you have the +1 Inner Circle strength.
Use this to go for enemies with toughness 4 and above. Make sure the unit has hero and magic items for backup. You need to win the combat res in the first round.
Check out the Griffon Standard 55 pts (double rank bonus) or the Banner of the Daemonslayer 50 pts (causes fear). Either (or both if taking a battle std in the same unit) of these standards could turn your knights into an enemy unit 'stampeder'.
