So you don't want to spend the silver to do a perfect fusion? You're probably wrong. The only reason it's probably is because to a certain extent, fusing rares perfectly doesn't really have a point since they are such a low value to begin with.
What is a perfect fusion?
A perfect fusion occurs when a player fully boosts and maxes the mastery of a card before fusing it with another of the same type that is also fully boosted and mastered. In Marvel War of Heroes Perfect fusions are necessary to hold the value of a card. Realistically, if cards haven't been mastered before fusion, the stat difference is negligible. The price difference of the cards on the other hand is substantial. This is partially because players have a perceived value of cards and perfection is so easy to attain that anything less is unacceptable.
Imperfect fusion is accomplished when one or both cards are not maxed in level or maxed in mastery. If the condition of both cards max level, both cards max mastery isn't met, this is considered imperfect.
Not maxing mastery
In Marvel War of Heroes not maxing mastery causes a mild decrease in price, if the player even notices. The stat difference tends to be so slim that most players won't pick up on if a card was or wasn't mastered properly. If a player does notice you can expect the player to either become completely disinterested in the card, or wanting a significant price drop, you can expect 10% or more in price drop.
Not maxing levels
This is the most awful mistake a person can make with a card. This is a way to completely ruin a card in most cases. For example, I don't care what the price is in most cases, I will not buy a card that has not been boosted to its highest level before fusing. The reason for this is boosting a card to max level will yield 10% of its stats carried over to its final forms, even one level less than max will reduce it to 5% of its stats carried over to its final form. These cards just aren't worth the decrease in stats.
When to do/not do a perfect evolution
If you aren't doing perfect evolution I'm hoping you're dealing with something in the rare quality or lower. Anything in the SR, SSR, UR quality should always be a perfect fusion. In the rare quality I also often advise a perfect fusion since it will result in a much more powerful fusion and allow you to use the card longer.
Moral of the story
Fuse your cards perfectly if they are SR or higher. Anything less than perfection will lose a lot of potential buyers and lose a significant portion of price. Personally I'm sick of people trying to trade me imperfectly fused cards, because once it's imperfectly fused, it's garbage in my eyes.