The postgame FAQ lists the most efficient combination of items to upgrade weapons:
http://www.gamefaqs.com/ps3/928790-final-fantasy-xiii/faqs/59327
though some of the items might not be available for purchase yet.
Anyway I thought that FF13 was balanced well enough for the most part and was fairly consistent in difficulty throughout the game without going out of your way to grind or tinkering with weapons (well, besides chapter 11 where they wanted you to take your time before progressing the story, and postgame which is pretty ridiculous about grinding if you want Treasure Hunter). The only time I felt that battles were taking too long were the Sazh - Vanille parts in chapter 4 where you couldn't really hit the weaknesses of the mechanical enemies and Sazh's stats are pretty bad.
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The only certain death in the universe is Mother Hitton's Littul Kittons.
Not changing this line until I beat Seven Heroes (2/24/07)