Ive never done 5x5's as i dont really like the idea....but Ive mostly worked on higher rep sets...8's..10...12's. My theory is generally....if i can do 10's I up the weight. If i can do 8's but not 10's..i stick on it till i can do nearly 10's then i up the weight and so on. If i feel like upping the weight just for the ell of it..i do it. Do how you feel at the time...some days you are stonger and more energetic than others. It doesnt mean you arent gaining/improoving.