When contemplating whether or not to attack an opponent with a Terran Marine rush, it is important to understand what type of situations your marines need to be kept out of and which situations they will excel at.
This means understanding a good bit about race balance and what units the marines need to avoid to survive and be victorious in a Terran marine rush.
That said, here are 3 key points in which I would avoid the Terran marine rush strategy and shift to MMM or mech units instead.
The first circumstance to avoid a Terran marine rush is when you are playing on a novice map.
These maps are specifically designed to avoid allowing players to quickly rush up the opponent's base by providing blockage in the choke point that delays any attack from hitting them too soon.
I would recommend avoiding a standard ground rush at this point and instead shifting to reapers or MMM to get in at a good angle with your opponent.
Next, keep your eye out for units that provide an effective counter.
Do this by scanning early when you first start massing marines to use in your Terran marine rush.
If you see banelings, colossi, siege tanks or banshees, I'd recommend avoiding this Starcraft 2 strategy simply because it will cost you a little bit too much to attack effectively.
Finally, make sure your ally is available to provide support.
If you don't have an ally, take out the expansions first and move gradually inward on your opponent to prevent him from gaining a technology or economy advantage over you.
If your opponent has a thick choke that you cannot penetrate or a significant tech advantage, the timing is very bad for this particular strategy unless your sheer numbers are overwhelming.
Keep all of these factors for potential defeat in mind before going in with a Terran marine rush!