Price in absolute terms means very little when it comes to spreads. In the absence of some edge related to the calendar or shifts in the shape of the curve I would rather sell vol outright.
My advice would be to stay away from spreads until you find some term structure alpha and can recognize and pick off dislocations.
Isn't this alpha very fleeting. Requires you to recognize and at least have a feel for the probabilities as to how market prices will evolve. It's different every time.