You want to eventually expand your ear to learn what sounds really hip. Flatted 7th start to sound too standard after a little while.
One of the coolest songs that I've learned, Seven Come Eleven, has a major 7th in it, a G chord in key of Ab, but you don't really think of it as major 7th, you think of it as being borrowed from another key relative the chord after it.
So, the progression goes from Ab to G7 to C7 and you think of the G7 as the dominant fifth being borrowed from the key of C. It's seriously COOL when you land on a G7 after playing Ab lines!
Yet resolving to the C7 chord makes it not too dissonate as that is the III7 within the Key of Ab.