I agree that you should ideally use the running backstay anytime you are using the staysail. I will say we do use the preventers anytime we go greater than 100deg apparent. It doesn't take long and the downside is too big.

That said, we don't do everything perfectly aboard Tengah all the time. Regarding the running backstay, if you ever see the baby forestay getting loose from the staysail pulling the mast forward, you definitely want to use the running backstay at that point. 
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Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com