You can't determine pushrod length with any sort of accuracy until you've got the cam installed, heads bolted on, lifters in the bores and rockers snugged down. Then you really need an adjustable checker pushrod to determine what you need.
Millions of factory built engines would disagree with you.
Millions of factory built engines used the exact same cam grind and exact same design of lifter rolling down the assembly line, so this is a bit like saying there is no need to degree the camshaft when the factory just slapped them together dot-to-dot.
Assuming OP is not using NOS factory replacements, then the base circle of the aftermarket cam grind may be different, the manufacturers recommended preload on the lifters may be different, thickness of head gaskets used may be different. Pushrods are probably the most overlooked part of an engine, and probably one of the biggest reason builds have disappointing performance or noisy valvetrains.
