Any "strategic" way of using the sources

[DeadReD]

Mind source/speed source etc..
My first time I just used them randomly on whoever. Wondering if there is some sort of strategy to effectively assigning these to chars

[Wolf Kanno]

Not really, your equipment will have a much greater impact on character performance than the Sources, the Ultimate weapons already allow for max damage at even low levels, and hell the overflow glitches in Barret and Vincent's weapons allow them to 1hit KO the WEAPONs.

At the 60 level range, you're already going to be faster, hitting harder, and taking little damage from attacks to bother with the Sources. You would need to farm a significant amount of them to see any real impact on your characters. Even then, Magic Sources are probably the only ones worth getting cause Aerith is your only strict mage character, with everyone else playing second or third fiddle to her. Even then, the damage output of later summons, some of the Enemy Skills, and just the Ultimate weapons in general make trying to improve a characters Magic or strength stat rather pointless.

I'd just ignore them, unless you want to take the time consuming task of farming them until you max out all your maxable stats.