Is it better

There is no evidence ( in my experiance or in any proven experiment) that value of shards impacts reroll odds. So do what's best for your account. Low or? Disenchantment. Got a fat stockpile? Hope for rng to shine on you.

Speculative, I suppose it depends on how often you get skinshards, and how many champions you regularly play. My rerolls often ended up being for champions I either didn't have, or don't play outside of aram.

Personally, and I know some other people who do the same, I only re-roll skins that are below Epic tier. Epic tier shards give a decent amount of OE, and with a few of them you should be able to unlock an Epic shard permanently, whereas lower tier shards give so little it's generally better to just try the luck.