It really depends upon the spell. Some (such as Firebrand, The Symbul Matrices, and others that don't do things that have changed) are usable as is.
Others utilize game mechanics that have changed. For instance:
Battletide: Grants 3.0 Haste effect. This shoul be updated for 3.5's haste.
Harmony: Says what to increase the bonuses of bard song to. it should instead just increase them by +1/+2 since Inspire Courage has changed majorly.
Basically, using any 3.0 resource in 3.5 reuires at least a momentary perusal of the item to see what things it may do that have changed in the editions. The vast majority of things will be fine.