Not sure if this will help but check out the link below for devices being out of order.

http://fcp.surfsite.org/modules/smar...php?faqid=2617


Concerning the WB each has there responsibilities to perform so when all are using WB "a means for Write-back caching" - it will perfrom better for each component transfer. So I really do not see a danger with setting these options. IMHO