Why 5? 4 caps you on CDR and gets you full stacks of BC on the first physical damage you deal. Any more is wasting CDR and doesn't make you stack it faster, if you want MORE pen you can grab a LW. Though you should probably buy a GA or something since you're going to be rather squishy with just the pure HP you're getting.
If you read this signature, then that meant that I had control of what you read for 5 SECONDS!! http://imgur.com/n8yh1
I built 2 in my last ranked with Garen. Until they make BC unique, 2 is the way to go. I wouldn't build more than that, especially since relying on pure health for tank is dumb with the new mechanics and numerous % health damage items.
And all the feeders and trolls will look up and shout "Carry us!" ... and I'll look down and whisper "No"