Shamelessly swiped from Gareth.
Hazarding a guess in an attempt to complete this answer:
ROSE. (Appropriately for Valentine's Day.)
in history, it was assumed
i possessed only five
Wild roses only have five petals, and indeed the flowers of most rose species have just five.
a change in tense, a compound word
you'll see me come alive
rose → rise (change in tense) → sunrise (compound word):
the sun coming alive
a skyward gun, ...
... may reference the pistil, the female portion of flowers including the rose. Different species of roses have different numbers of pistils, with most having several. While its name sounds like a gun, a pistil is nothing to be afraid of.
... but don't be scared
No need to run off! Stay, you guys! Stamen! (Yesh, it's a stretch. Humor me.)
your judgement's preconceived
Judgement may hint at stigma, and preconceived may reference the unfertilized egg in the ovary - all parts of the pistil which, together with the stamen, are the reproductive systems of the rose.
the best in life appears to those
when through these it's perceived
Rose-colored glasses, of course.
If this is correct, @Gareth McCaughan should also get an upvote for shoving us quite far in the right direction.