Long sleeves in the summer time?

Okay this is something that I am very conflicted on as I choose clothing for our kiddo's first year. Having a July baby, I am constantly urged by my SIL to register for nothing but long pants, coats, long sleeves, no skin exposed whatsoever clothing for our newborn. She too had a July baby and wants to pass on her knowledge which I absolutely appreciate. I have a friend who just had an April baby last year and she goes by what her kid prefers and is comfortable in.

I have always heard to always dress your baby as if it was 10-15 degrees colder, or if you're cold then baby is freezing. I've also recently heard "cold babies cry, hot babies die." With all the conflicting information, which do I believe?!