It's a good idea to snap off those yellowed leaves where they meet the main stalk (not cut), as long as they are beneath the first flower truss. Once they are yellowed they no longer manufacture sugars from sunshine plus chlorophyll, so all they're doing is blocking air circulation around the plants.
I know because I got a comprehensive bollocking from our flower shop lady, who grows large scale tomatoes, when we first moved here

If it's the top, new growth which is coming in yellow, then that's something else and we would need a pic.....time we had a photo icon on TAS
