I use the simple rule of thumb that if I have plenty of dormant grass or am putting out enough hay, cattle will get enough energy, given proper protein supplementation. If you don't have enough dormant grass or hay, you'll be better off using a natural protein supplement that will also provide some energy, such as range cubes/cake, a better quality hay, etc. That said, I always prefer natural protein and don't mind spending a little extra to get it. With a tub, though, you'll never get the high protein levels without using the NPN's.
Basically, if you have enough dormant grass, the NPN/urea supplied in a protein tub will help supply enough protein for them to get the energy they need, but you still have to answer some more questions to know if that will be enough. For your purposes, the question you have to ask yourself is whether or not they can consume enough of the tubs to get the protein they need. That is going to be dependent upon whether or not you're talking about yearlings, wet cows, dry cows, etc. With dormant grass, it's often difficult for a wet cow to consume enough protein from a tub to give her what she needs.
Protein tubs are the poorest bang for the buck with your protein dollar, though. I see a lot of protein tubs that cost close to $1,000/ton. As much as I'd like to use them for the convenience, I just can't justify that in most cases. But in some cases, we don't have choices, and if you're not out there every day or at least once per week, you might have to bite that bullet.