When formatting a RAID device with an EXT filesystem, it is always advised to specify a stride size. The format utility will take this stride size into account when formatting a device. The stride size is the number you get when you divide the 'chunck' size, as specified with MDADM by the filesystem block size (almost always 4K).
So a 128 KB chunck size gives you a stride of 32. A nice and simple utility to calculate your stride can be found here:
Please note that the stripe-with option does not seem to work on Debian Etch. Maybe because that option is too old or too new.
I also think that most recent tools automatically detect and calculate the correct stride size for you.