Thankfully we are talking about a game here, a game that allows us to practice handgunning techniques, but a game none the less. The issue of shooting over cover versus around has been hashed about for years. The fact is, is that they both have their uses in the "real world". Going around cover is good, in that you are better able to utilize whatever it is that is stopping the incoming while minimizing exposure. It is bad, however, when facing multiple opponents due to the tunnel vision effect and a tendency to focus on the primary threat. Focusing on the threat to the exclusion of the periphery will cause you to expose your flank.
Shooting over the top of cover, barrels, mail boxes, car hoods etc. gives you the opportunity to view movement on the part of the knuckleheads, and the ability for you to react, but it also gives those same knuckleheads a target to shoot at.
As a general rule knuckleheads tend to run in groups.
But, what we are talking about is a game, if it isn't written specifically in the COF do whatever you think is best.