this reminds me... must buy a bottle of brake fluid before I start working on replacing the front hubs and all 4 rotors + pads....
In my opinion it's better to bleed the brakes at every pad change. You may find that your brakes weren't as good as they used to be not just because of the worn pads but also an air bubble or two. I figure, while you're working down there you might as well do it. Oh, and it will cost you very little extra to buy the fluid.
It's important never to reuse a bottle of brake fluid. Use it once and if there is anything left over once all the topping up is done on the day you changed the pads, then toss the bottle (responsibly of course). Opened bottles have their seal compromised and this can lead to condensation and water contaminating the brake fluid. Not only does it change the hydraulic properties of your brakes but it can also lead to corrosion and damage to your brake system. Don't risk it. Buy new fluid today!
tehehe (all seriousness though, always use brand new fluid for the aforementioned reasons)