It depends on what you are doing as far as cow aggressiveness or expressiveness is concerned. In the cutting pen back in the early days judges liked to see an aggressive horse with his ears pinned. The trend now is an expressive horse with his ears up. Cutting trainers actually like a colt that starts out a little scared of the cattle, that makes for expressiveness and suck back. Don't need to be pulling him off when he leaks into the cow, or pounding on him with the cow side leg to keep him back.
And a horse that is scared of cattle at first doesn't have anything to do with whether he will have the natural ability to watch a cow and work it. That comes with confidence building. Same with one that may not show much interest, sometimes it takes some time and again, confidence. Working for trainers and starting these types of colts I usually turned back on them in the cutting pen after about 15-20 rides. Some would try to buck and run off, others lazy and uninterested and some aggressive. After time(some take longer than others) they all developed and shown what they would be best suited for.
Personally, I like the aggression. I think they make for snappy horses for heeling and awesome for going down the fence and circling in the cow horse, however it makes boxing a little of a challenge.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.