If you want to get the most out of your units, it's best to know how they work!
All weapons in the game have a range, and you can conveniently see the range of the weapon when you hover over a unit while holding shift.(depending on your settings: Defaults may vary and your settings may vary) This displayed range is when you consider the weapon firing from the center of the unit. This displayed range is quick to display and gives you all the information you really need about weapon range. Most units, however, don't fire from their center, but have a cannon or other gun mounted to their frame and fire from that. The game really cares about it's physics system, so weapon projectiles will fire from the end of the barrel of what ever weapon is on the unit, and travel from there to it's intended target.
What that means is mirror matches between units, even with instant hit weapons, can have a 'favored attack angle.' For venom, for instance, you can get first hit advantage if you are 'retreating' away from the enemy, as your turret is closer to the back of the unit. Exhibit A1 and A2
lance better corner peeks with it's aft rather than it's bow. Exhibit 2
Glaives are all left handed, meaning back glaives like standing behind and to the left of allied units. Exhibit 3