A K-1 is not a "Try before you buy" visa. To file an I-129f you MUST submit a letter showing commitment to marry. Why wait?
The US immigration process requires a great deal of knowledge, PLANNING, time, patience, and a significant amout of money. A wise couple should have the entire process locked in and planned. Besides, waiting 60 days just delays the ability to work, delays the ability to travel outside the US, delays obtaining the Green Card, and delays the ability to apply for citizenship. I wouldn't wait unless there were real issues between the couple.
Just my opinion.