I replaced my FIs with new Ford/Bosch Type-IIIs from Five-O.... I have not (nor will I) flow-tested the old ones, but they were pretty crudded-up.
I recently sent my Toyota injectors to be cleaned & flow balanced, and I'm glad I did. 3 of 4 were fine and within 2% of spec, the fourth was dripping and about 5% under spec. After cleaning - the worst of the four is 0.2% off spec (479 cch at 43 psi, instead of 480 spec). I'm lucky, the 5% under injector was pushing the limit on AFR in one cylinder, that could have ended up costing me my engine.
Moral of the story - if they are clean, keep 'em clean; if they aren't clean or you don't know if the po used any cleaner, either get them cleaned or replace 'em.