Will assume you're talking about the 'Column Browser'. There is no way to change the default "All" to some individual genre. Well, there might be with a text editor and knowing how and which file you need to edit. But, that's way beyond my expertise.
The easy way to do it, don't turn iTunes off. If you're like most of us that only reboot the machine when it's required due to a software update, you're talking about only once every 2-3 months or so you'll have to be bothered by selecting the genre.
The presentation of the iTunes Store is controlled on Apple's end. So, short answer, you can't change the default presentation when you log in to the iTunes Store.
Besides, seeing stuff outside your normal comfort zone is GOOD FOR YOU. Now eat your vegetables.
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