Firstly, this is what I've done for the past 7 years.

Create a 728x90 advert, place it in the header.
Create another 728x90 advert, place it in the footer.
Create a 300x250 advert, place it on mobile footer.
Create another 300x250 advert, place it in the news item.

Create custom channels for each advert.

My question is, do I need to create two seperate 728x90 adverts, or should I just use one? I've always created two because I like to name on Header and one Footer. Likewise for the 300x250, I always create two even if I'm going to use that same size advert twice.

Is there a correct way here or does it not matter?