You will find your time more fruitful focusing on leveling a strong roster of four stars… those five star ascension mats due to randomness of drops will create a choke point for levelling five stars anyways.
If you want a northstar for leveling strategy… the 2 priorities should be
- roster depth
- hero synergy
So focusing on a deeper four star roster to create more synergistic teams for raiding and titans will benefit you by yielding better scores and loot drop chances. And You will get to enjoy another aspect of this game by getting to participate in alliance wars quicker if you can build a war roster of four stars.
Some useful links that is nice to share (and helps with four star prioritization):
Titan and Raid architecture infographics (to help synergize your offensive heroes)
Defensive setup inspiration: