One of the big advantages of meal planning (which for us just means writing meal names on a list) is that it helps you remember when leftovers are from. Did we make macaroni and cheese Monday, or Tuesday? Check the list.
In most cases you only need to remember for a few days. Things you cooked and put in the fridge are not good for longer than that, so you don't have to be able to remember whether it was 9 days ago or 10, either way it's going to be thrown out. Sometimes I will put mashed potatoes in the fridge and see there are some from an earlier meal already there; I throw those out immediately so there isn't confusion about which ones are from last night and which ones are from over a week ago.
When it comes to items you buy, either it has a best before on it (milk, cheese, sliced meat, eggs) or you can tell by looking (vegetables, fruit, bread.) As a result I see no need to add labels. The one exception is eggs, which I get from the farm in reused egg cartons, meaning the best before on the carton means nothing. I keep a little slip of paper on the fridge, and when I buy eggs I note the date and a description of the carton on the paper. (Eg "blue Burnbrae CSA Jul 7th.) CSA stands for Community Supported Agriculture and it's where I get my eggs from. I would rather not write on the carton, because it's going back there when the eggs are finished.