This myth is partly true, partly false.
It’s partly true because selectivity varies by office at GMCs. Smaller offices may prefer a weaker candidate who ranks them #1 and has a rationale for that ranking (eg, it’s their hometown, they’re interested in the region’s dominant industries) to a stronger candidate who doesn’t rank them at all.
It’s partly false because selectivity can vary significantly in the smaller offices year-to-year. Particularly in the current hiring environment, smaller offices may make offers in the low single digits (if any).
Here’s your takeaway: if you’re a very strong candidate, it won’t matter to which office you apply. If you’re a borderline candidate, you may want to give office preferences a closer look – but only if you have good personal and professional reasons to do so.