Use visa waivers. They let you stay in the US for 3 months at a time and spending $1k on a ticket at the end of each 3 months is cheaper than paying $5-10k upfront for a visa plus additional flights.
If you are worried about the number of times you can enter on a visa waiver, don't - I have entered the USA 50+ times on a waiver and the guys at immigration ask me how I am going and wave me on each time.
This was even more fun before they closed 'The Mexican Loophole', where you could drop out to Mexico for the weekend and get a new visa on the way in. I had a lot of fun weekends in Mexico before they shut that down. You now need to leave the North American continent (they included Central America when people started flying to Belize to get new US entry visa's)
Visa waivers (ESTAs) are fine during YC, but after 6-9 months it becomes necessary to have legal status to do things like fund raise (many investors only do local), paying yourself, staying in the country for more than 6 months in a year etc. Obviously there is also the risk of the nightmare scenario that immigration dude has a bad day and doesn't allow you to enter the US. Whilst this is rare, I know of it happening.
Most international YC companies end up moving to the US, and they sort out visa issues after demo day as it takes too much time during YC.
The list of countries that you can get a waiver from is a lot longer now than it was when I started doing it. The USA realized that they depends on business tourism/migration, so they have been steadily expanding that list of countries and the scope of the waiver program.
I know of two people who did something like that and it went wrong. If you are not lucky you get the "wrong" immigration officer and then you won't get into the US anytime soon.
If you are worried about the number of times you can enter on a visa waiver, don't - I have entered the USA 50+ times on a waiver and the guys at immigration ask me how I am going and wave me on each time.
This was even more fun before they closed 'The Mexican Loophole', where you could drop out to Mexico for the weekend and get a new visa on the way in. I had a lot of fun weekends in Mexico before they shut that down. You now need to leave the North American continent (they included Central America when people started flying to Belize to get new US entry visa's)