Adding weight when you get to a certain point is fine if you want to push yourself. However don't make the mistake of sacrificing form. I can't tell you how hard I laugh at the guys in my gym who pretend to get stronger by adding more weight when the really shouldn't. Example. Guys doing deadlifts this morning. They were doing more weight than they should but they want to think they are strong. There form sucked. Back was curling over as well as each rep was bounced off the floor. These are the same guys who do "curtsy" squats. Where they have 3 plates on the bar and go down about 4 inches, grunt and go back up. Just don't be those guys in your weight progression. Other than that find what works for you. It is mostly a mental game anyhow. So if you barely get your last rep in why not try next week for a little more. If you don't get it don't worry you tried. Maybe next week you will. If you want to wait a bit longer before trying again great. Maybe add another rep in and have your buddy, if you have one, help you squeeze it out. Sometimes you think you can't get one more by yourself and then all of a sudden you do and you have no idea how but you feel great. So just push a little harder and the progress will come.