I only have one iPhone. Should I turn on backups

Why would you think not backing up your iPhone would a smart thing to do? It doesn't matter how many devices you own, they should ALWAYS be backed up. There's nothing worse than losing an iPhone, and not having a current backup to restore to a new phone.

Turn off iCloud backup if (and only if) you would prefer to back up the iPhone using iTunes.

How to back up your iPhone, iPad, and iPod touch - Apple Support

iCloud is an NOT an archive system. If you delete your data on iCloud, your data will delete on your phone. So, not a good option at all. Unless you want to backup your phone to iTunes as sberman suggested.

If you have the free 5gb iCloud storage plan, wouldn't it make more sense to just buy the 50gb plan for a mere $.99/month? That's less than two specialty drinks at Starbucks and 50GB goes a long long way. You can upgrade the storage plan for iCloud so so easily.

Click here for details on how to upgrade your iCloud Storage plan: Buy more iCloud storage - Apple Support

Instead of keeping all those photos on iCloud, use a free photo backup service from Amazon or Google or just upload the photos from your iPhone directly to your computer, then delete them from the cloud. They'll be all safe on your computer and you'll reduce the space in iCloud considerably.