You need to use the same scale because each scale will read a little different. I have 2 scales and they seem to read about 3 lbs different. As for how often to weigh in I say do what you feel comfortable with. I myself weigh in daily. I like to know if what I am doing is causing water retention. I know there is NO way I am going to eat enough to really gain 5 lbs in a day, but it makes me think of what I ate and drank the day before so I can understand how my body is reacting.
Truthfully I find that weight on the scale isn't always the truest measure of where I am at in this process of getting healthy and reshaping my body. I find that my weekly measuring is a truer indicator of my progress. Picking key areas like your neck, chest, waist, hips and thighs to measure with a cloth measuring tape will help especially if you are building muscle as muscle is more dense and takes up less space.