This advice is intended mainly for large games with 6 or more players on your team. Smaller games are more concerned with individual factory matchups.
Grey: Place your Commander near metal spots that nobody else is near.
For your first few games, you're expected to spend more effort learning more than contributing. But making sure all the metal spots in the start box get their Metal Extractors quickly is a quick and easy way to help your team start strong. Afterwards, being slow into a lane means your teammates are defending you while you experiment with different units and figure out what your army should look like and where it should go. These slow starts are bad for a higher rank player, but you shouldn't expect to play like a high rank player yet anyway.
Red: Place your Commander and Factory where it can support a teammate.
You should've figured out how to fight with some units by now, but probably won't have a response to everything the opponents do. Shadowing a more experienced player lets that player handle the particularly tricky situations while you watch, and lets you focus on specific unit matchups with the teammate balancing you out.
Brown: Place your Commander in the front where you can immediately push forward for territory.
Teams get large advantages when they control more metal spots after the initial opening push than their opponents. You start with a Commander that can both build and defend Metal Extractors. And territory is easier to defend than to assault. Expanding forward quickly at the start is one of the best things you can do for your team.
Copper: Aim for territory near the midline but of modest economic importance.
At this point you should have the skill to hold your own on the front, but only if the other team doesn't focus on you too much. You can take advantage of this by staking a claim on territory which it's a mistake for the opponents to focus too much on, but that's still an important enough link in your team's frontal defenses to dedicate a full player to.
Gold rank tends to polarize into two playstyles and my advice for each is different.
Gold Specialists: If you're the type of gold who tries to practice one strategy to perfection, place your factory as early in pregame as possible.
This lets your teammates plan their own openings around what you will do and greatly improves the odds you'll get effective support.
Gold Generalists: If you're the type of gold who is collecting a broad collection of strategies, place your Commander late in the pregame to balance your team's numbers between fronts.
Your resources should be somewhat useful anywhere, but your aren't critical anywhere. But the team losing one front to being badly outnumbered there or to having one player responsible for too much frontage are real threats. So, keeping in mind that some fronts are longer than others, put your Commander where the team's frontage-to-body ratio is lowest.
Silver: Be ready to change your plop to fill gaps in the team's composition.
You now have the game knowledge to look at the whole pregame board and figure out what's missing. That could be a specific factory that the terrain demands. Or it could be an area with insufficient player coverage, as with the gold generalists. But at your level, it can also be a comparative skill void with a mass of only brown or lower ranks.
Blue: Choose your starting position to be able to bail out low-ranked teammates.
This is no longer about where you start, but where you can go from there. Where Silvers can relocate in pregame to be "lane leader" of an otherwise unskilled lane, your team needs blue "front captains" who have a main lane or gameplan each, but can quickly reinforce or rotate into any lane that doesn't have at least a silver. If your team has few blues, then plopping on the map edge is suspect because it means you can only sidestep in one direction, but that might be worthwhile if it means you're in a position to back up an otherwise exposed gray or red ranked ally.
Purple: Choose your starting position where you can achieve a game winning advantage.
By purple you should have a better grasp of pregame than I do. But the one pregame mistake I do often see is to proceed with an initial plan that has significant but ultimately limited upside. For example, a purple can win the sea completely as ships on Folsom, but without the momentum to push ashore they risk losing the rest the map to skill differences before any fac switch could make a viable land army.
