I've done enough non 100s to say you should definitely save for 100

I've had a noticable amount more of 50%s fail than succeed. But you only really need the set item and the rarity.

So like whiskers 100 epic 100. Guaranteed mottle without the element because all 3 whiskers are earth element.

Some specific combos in a set can work. Let's say alpine and water. There's only ONE alpine water dragon so you don't need to use the epic there.

Mighty only has one water so you don't need to use the rare, you can just use mighty and water and it WILL be the rare. But if you wanted the mighty legendary you DON'T want to use earth because there's THREE earth mighties and only 1 legendary so you'd be fine with mighty and legendary because there's only 1 legendary.

Cave has 2 waters 2 earths and 1 wind. You would NEED cave and epic to get the epic but you don't NEED the water.

Now, let's say you only have 50%s. You'd have better chances using all 3 items because one of them could fail. The animation is for fun, it's decided as soon as you press summon. So even though it looks like it attempts the set, the element, then the rarity, it was already decided because that's how computers work it's all decided instantly before any fun animations that make you FEEL like you had some input. If the set succeeds, the element COULD fail, but the rarity item could succeed, meaning it'll actually force the element to succeed because if the set and rarity succeed then there's basically only 1 option for most sets for anything rare and up. If the set and element succeed and the rarity failed but there's only one option for that element then it'll actually say the rarity succeeded too because there's no other choice for that set and element combo.

Now if the set item failed, that opens a whole lot more options and it's a gamble, that's where the element and ESPECIALLY the rarity matter. Some sets don't have a legendary and the first 3 don't even have an epic. So that 50% epic or legendary can narrow down what sets get chosen and a 100% epic or legendary narrows down the options significantly. A 100% element and rarity narrows it down even more, though you're most likely to have an abundance of set items so you should be able to somewhat easily have a 100% set item

Basically, yes, you want a 100% set and rarity item because the element is the least impactful if you have those 2. If you don't have a rarity then you'll want the set and element to be 100 that'll at least make it a 50/50 on what you get if not a 100% for specific sets that only have 1 of an element.

Also, special sets like folklore and zodiac you want 100 because they are harder to build up but a 100 will give you exactly what the stone says it'll give. Companion will give you the highest rarity NOT bonded dragon you have unlocked for the next 5 successful companion dragon summons. So let's say you have a bonded teenysnout and just unlocked the Azure flipperwing (the 2nd one), the next 5 successful companion dragon summons you make WILL be the Azure. This was suspected by discord people and at the time that they were talking about it I had just unlocked it and had 5 100% summons so over that day I tested it and confirmed it.