I have a complex configuration of multiple ADSL connections from multiple ISPs being load balanced by a Draytek Vigor 3300V+

My question is what DNS settings should I use. Should I use the ISP setting for connection A or the ISP settings for connection B or something different?

I am currently using Google DNS settings 8.8.8.8 and 8.8.8.4 on my routers and draytek but I fear that this might be the cause of some problems. Can anyone advise or point me in the right direction.

PS As soon as Fibre reaches my office I can abandon this complex setup