Just my opinion:
What you play is more important than the humanizing of the timing. Although I am not necessarily the perfect example, I try to think about a human playing the instrument and how he or she would play it. This is extremely critical in guitars, violins, etc.
For drums and bass, listening to records and paying attention to what is played is very helpful; but keep in mind that a very large percentage of records use MIDI bass and drums.

Rock and country are more likely to have real players than pop, but there is no guarantee. Older rock records can be a good source of licks and styles.
Drum and bass VIs that have round robin samples—that is, playing the same note repeatedly will cycle through several similar but not identical samples—and VIs with mutliple velocity layers sound much more authentic. If you don't have that, you can still work around it with good playing.
My biggest weakkness is bass lines. For some songs I write, I come up with bass parts I really like. For others, I can't seem to get an original motif.