I just made an observation: If you choose a new dice for completing a level you can sometimes find a mundane attack or defense dice that have a slightly higher percentage or rolling 6 as you can see. Now if you add the face values together and divide by six, you'll be able to see the average power of each dice. The standard defense dice at top tier (4+4+5+5+6+6= 30/6 = average value is 5) whereas the other dice (6+6+6+5+5+4= 32/6 = average value is 5.3). In the case of the defense dice this becomes interesting when compared to a top tier Lucky 1 dice (6+6+6+6+6+1 = 31/6 = average value is 5.16). My conclusion is that if you can choose which dice to put a Grow tag on, chose the mundane one you get as a reward. It will, on average, outperform the starter dice, and if a Heavy tag is applied to it, it'll even outperform the Lucky 1 with Grow.