In general, breakfast is likely to the least expensive meal of the day,
but it does depend on where you plan to eat. I wouldn't necessarily say that I'll always pay OOP for breakfast and use my TS credits for dinner regardless of where I am eating. For example, if you opt to go to Akershus for breakfast ($41 per adult) and Garden Grill for dinner ($38 per adult), it makes more sense to use the
DDP credit at Akershus and pay OOP for Garden Grill. Basically, if you're doing at (character) buffet breakfasts, those might be more expensive than a non-character/non-buffet meal.
Some of the TS locations have fairly inexpensive menus (for Disney), so you just have to keep an eye on the prices. In general, the a la carte TS breakfast locations are not a good use of credits (no dessert), but the breakfast buffets can be an okay use. So if you're planning on an a la carte TS breakfast, then I would probably pay OOP for that and use the DDP credits on lunch or dinner.