I'm usually bound by client meeting times, and trying to get home soon. I never understood the Sat/Sun-night is cheaper rule, because I always included it. But now that I travel way too much for my family life, I actually understand it, and try to be home as early as I can, and I find myself willing to pay extra; in the same line, I tend to book my flights extremely late, more in case of home 'emergencies' ('sports event that I would like to attend' or 'theatre performance'...) than anything else.