Well, maybe your credit rating is affected by credit card inquiries, but I suspect it would take a fairly large number of card applications ot have much of an impact, not one or two a year incurred by adding cards for mileage.

In other words, the truism would apply to people who have made 20 or 30 credit card applications in a relatively short timeframe and not people who apply for a new card once or twice a year.

FWIW, I have applied for credit cards for mileage purposes once or twice a year and it has had little or no impact on my crdeit rating.

FWIW 2, you can get your FICO credit rating at the web sites for credit bureaus like http://www.experian.com and see for yourself!