It depends. With agility, there's a lot of running and hard work and I think its healthier for my Dobe to not have a full stomach. When he was young and beginning, I would make a conscious effort to make sure his tummy wasn't full so he'd be more attentive and the treats became a higher value motivator. Once he got into the "groove," it really didn't matter anymore. He enjoys training and being a participant. The treats are used still, depending on what we're doing and they do increase in value if he's a tad hungry... if it's between meals, which normally training "sessions" are just because it happens that we train in the afternoon usually.