As long as replenishing those R's is trivial to you, then go for it. The 'cost' of expending multiple R's versus an SR fluctuates depending on your supply of them.

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I should say 'worth' rather than 'cost'
Replenishing R's basically costs Gems and your time spent rolling the gem gacha over and over again plus donating normals to enhance your union's guardian.
Efficiency comes into play when the 'cost' (gems and time in this case) is non-trivial to you. Otherwise, if the above is trivial, then by definition you're not going to care.