Carbon is useful for getting any organics out of the water, so if you have normally stained water it might be worth running all the time.
Personally, I don't run it and have a bag on hand for use only after using a medication. I think I've had the same 1lb bag for at least 12 years now.
I do suspect it's cheaper in bulk than if you buy it in a bag with a picture of fish on it. So long as you get 100% carbon that isn't treated in any way, you should be alright. Might want to test it for the first time on some less expensive fish just in case.