The bias on an output tube is its DC operating point so the load on the secondary side of the transformer will have no effect on the results when using a volt or current meter to set it (Because a transformer will only pass AC signals). Even if you were using a scope to check for crossover distortion I don’t think the load on the secondary would have much effect.
Just to add you do need a load on a tube amp even a mismatch within a factor of two a resistor instead of speaker ect.