Well, I'm not aware of public empiric data on this so all we can do is speculate, but regarding the crit scroll there are a few things to note:
a) the effect is capped at tier 7 (I think)
b) higher tiers have a lower base-critchance
c) on lower tiers crits often yield 4+ relics and shards, at higher tiers it's rare to get more than 2 relics on a crit
In the end it's a matter of balancing shard rewards, relic supply, gold cost and time required. Personally I tend to combine all relics up to tier 5 once in a while, and then all but about 30 of each up to tier 9 (haven't done relic stuff since HD). The shards from crits combined with shards from crafted relics are generally enough for my needs, but that requires a large supply of relics (several hundred/thousand at tier 5) and gold. And if I really need some extra shards I have enough spare relics to refine.
Used to play: 85 Champ / Captain / Runekeeper / Guardian, Guild Master of everything but cooking.
Playing now: Hellcat / King Tiger / GW Panther / IS / KV-5 / M4 Sherman and more