Is it written that a 30PSI residential hot water boiler pressure relief valve must be mounted upright, or is horizontal OK?

I've read that the weight of the lever plays a role in the tension on the spring, and am confuzzled as to certifications. I've got one that's professionally installed sideways on a "T" that comes out the supply side of the boiler, and the plumber tells me it's fine and won't come back. As it's a safety device, we're concerned.