Synergy does have importance - this helps cover one unit's weaknesses whilst getting defended themselves.
Something like Skirmishers + Riots.
or assaults + raiders.
Artillery + Raiders...
But almost always it's more on the tactics; are you going to attack head-on? perhaps funnel the opposition into a trap? Wage an attrition war? Fight small skirmishes?
What strategy will you use? Will you be shock and awe, obliterate them before they have a chance to attacl, or will you be a siegemaster, slowly pummeling their defenses?
It's better to go into tactics/strategy than to go to synergy units, mostly because as Bass said, it's on the gamestate and opposition composition of units. This is because tactics go into every factory - synergy does not, most tactics you learn can be put onto another factory - certainly not the exact same, the execution will have to be pulled off differently, but the spirit of it is the same