Bill,
normally the preceding stage and the post stage will have some specification or limt. often expressed in terms of "reflection coeffcient" or vswr etc. Eve tohugh the exact impedance masy cahnege from say 42 to 71 ohm s in a 50 oh system, they beith prodce a -10 db (or so ) return loss. There fore the level of reflections and also the mis match loss associated with the random stacking of high output amplifiers (leaning toward 75 ohms) and the "loads" leaning lower toward
25 ohms worst case, and the an exac t 50 ohm amplifier loaded inot the perfect 50 load...Some time the "circle" of impedance acceptanace gets very wide.. then the design gets trickier. The analysis of often dome on compute, with both worst case and statisitc Monte carlo analysis completed for designs that will have huge volumes......
Best regards, Marc