18/21 is an excellent build from what I've seen of other pikers.
I personally have a different pike build simply because I have to perform different roles when I fight with the rest of Chaos.
BUT, on the whole, a dedicated piker build of 18/21 or 15/24 are excellent choices for a piker build. But I personally would push the 18/21 build, simply for the PowerStrike.
Your problem as a piker will be that your build will be next to useless when fighting alone, unless you're flippin' all over the place. In which case, you cannot risk a glance. 15STR will have you glancing maybe 20% of the time (1 in 5 strikes).
18STR will grant you 6PS, which will almost eliminate any chance of glancing. and 21AGI will allow you to head for the hills if your battle buddy drops dead.
Remember that it's important to be aware as a piker, I still get caught up in footman killing that I forget there are horses charging around the map, too.
Also, IF is optional with a piker build, simply because you'll be in the second line when your attacks become deadly. So people won't be able to get to you too much. But it can't hurt to have some onboard so you don't drop dead when you get scratched.
Best of luck! Please post more if you have questions!