I have two iPhones in the family - 4s and 5s - and they're both updated to IOS 7.1. I've noticed that both phones use the cellular connection instead of the WIFI connection when we're at home. For now I am disabling cellular when at home on both phones and re-enabling cellular when we're not at home, but I don't want to always have to do this.
Is there a fix to ensure the WIFI connection is always preferred over the cellular connection when connected to WIFI?
What happens if the phone is sleeping the wifi disconnects and cellular takes over. The only solution is doing what you are doing.
It really is not that much of a hassle.
I also turn off cellular and data roaming while out and about so as not hit another providers towers.
Your iPhones like all cellular phones will connect to do certain updates from the carriers this uses cellular data, and even if cellular is off per se the device still receives it's update anyway. Crazy I know but it happens.