I do agree that its not necessary to create DP90 if we decided to go for fixed billing by maintaining Billing form. But even user does mistake, system should not allow him to do the billing.
I maintained following settings.
ZXDR LEIS SENI L2S
ZXDR LEIS SENI L2W L2S
I have removed Billing form from DIP profile. I created a Service order with Billing form as Fixed Billing. I ran DP90. Still I could do the billing & Item category derived at Billing request is L2W which is allowing to do billing.