Yeah, that's hard to say based on the board and your field of vision.
I tend to take two cannons and spread them out. Each covers a somewhat diagonal arc across the board (if possible) with the knights a little forward and off to the side. Most fliers will try to come straight on at the warmachine so you can react with the knights to intercept, even if its shuffling sideways to prevent a turn 2 charge (and blocking LOS to the cannon).
If you're fielding an Engineer (recommended with a Helblaster), you can also give him a Ruby Ring. A little fireball action does wonders against skirmishing warmachine hunters since there's no minus to hit with shooting. Another ding against crossbows / handgunners.
Don't worry too much about keeping them alive the whole game. Prioritize your targets and take down as much as you can in the first half. Even if they keep big meanies hiding behind cover, they're doing a good job. I actually find that they kinda sit around in the later turns because the pickings are slim. Maybe finishing off a fleeing model or two just for the points.