Pushrods bend in the center, tapered with reduced diameter ends don't affect stiffness - just lightens your wallet.

Extra pushrod weight does matter, but much less than on the valve side, it varies inversely with the square of the rocker arm ratio.
Example: the effect of pushrod weight with a 1.7:1 rocker is equivalent to 34.6% (1 ÷ 1.7^2) of valve weight.