That may depend on how you are using low voltage alarms. Do you actually end your flights on auto-descend? Or is it more of a last ditch backup? If the former, you want it set at the voltage you want to land at, if the latter, you can have it lower but should have a system to know when you should be landing.
I have mine set at 25/15 %, but I tend to always end me flights close to the take off point, so a lower 2nd warning is within 100 Meters or so, plenty of time to get her back plus it gives me a couple of more minutes of flight time. I never let the batteries get below 10% except for the 1in20 conditioning.