ticket booking for visitor visa

[urban2005]

I am planning my parents to travel to US for 6 months. They are on visitor visa. This is for second time, and I have gathered that you can stay in US on visitor visa for 6 months max, hence I was curious whats the ideal to get their tickets booked. I mean should I book it for round trip (it helps as tickets are cheap) or should I wait till they get their I-94 and check the expiry date on that and book their departure ticket accordingly.
I will appreciate any thoughts on this.

thanks

[immihelp]

Get round trip tickets for their intended duration.