I'm a fairly new tractor owner, and there are more experienced people here who can advise you -- but I've done a lot of research on the forums (fora?) and here's what I think.
With a backhoe or a box scraper on the back, the additional weight will do a lot to offset the weight of a loaded FEL up front. That said, 3-pt attachments are unlikely to do as much to promote stability as filling the tires, which will lower the tractor's center of gravity, thereby making it more stable. This comparison is probably more true for the backhoe example ( with lots of weight up high) than the box scraper example (with the weight down low), and I personally would love to have both the boxscraper and the filled tires. It's tough to have too much stability -- especially when you have slopes.
I had my tires filled. Just my $0.02.