Will store cards affect my credit?

I'm 22, just graduated college and have a job and internship. My father issued me a credit card my freshman year of college just for food and such. He paid it off every month, therefore I have a credit score of 750. I applied for an Express Credit Card the other day because I wanted to receive all the perks. (They have a really good rewards program) and I shop there a lot anyway so I figured it would be worth it. I got approved for $750 spending limit. I spend $91.65 and paid it off within three days. (I would have paid it off on the first day if I had the cash on me) Now I want to open another store card, at Radio Shack. I don't even really want to buy anything, I just want it for the 15% lifetime battery discount, plus I get 10% off my first purchase when I am ready to buy something. I used to work there and I am into electronics. My questions are as follows. Will opening two store cards negatively affect my credit if I am instantly approved? Are these still considered "hard credit checks even if I am pre-qualified?" Also, I plan to pay my purchases off on the same day as I buy something at either Express or Radio Shack. I know that these cards will stay on my credit history for two years but they really only matter for one year. However, I am not planning on getting a mortgage or a loan anytime soon. So will this help me to build by credit even higher in the long run? Bottom line: safe or dangerous?

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