The only way that amp could NOT need rebiasing is if it's cathode biased - if so it would have an output around 20-25 watts and a very early Fender "tweed"-ish tone (FWIW even cathod-biased amps need to have the bias checked periodically, because if parts drift you can end up in big trouble, as they run hotter than a "fixed" (which is really adjustable) biased amp.

If it's a fixed-bias amp it may be designed to be tolerant of variations based on different tube applications - but for optimum tone and health of the amp you should always bias a fixed-bias amp, no matter what the manufacturer says. Soem make the "no bias" claim based on use of a certin tube type and rating system - go outside that, and biasing is mandatory.

Simply - ignore the instructions and have the bias checked and adjusted for safe operation AND the best tone.

(Years ago we never...or rarely....biased amps, as the tubes you got from the drugstore were consistent. The whole scenario has changed now, and tubes specs among the same types vary widely - some aren't even what the label says; Sovtek 5881's aren't a 5881 OR a 6L6 or even an audio tube, but instead a Russion servo tube with similar specs; and the early Sovtek 6V6's were "6L6"-ish tubes that failed some tests, but would pass 6V6 requirements!)