Lifting weights – Should I still breath on each repetition?

Yes I would do it on each repetition to start with. But ALWAYS with closed mouth – also on exhale. You are making a lot more CO2 during the big physical strain as weightlifting. Don’t bother about hyperventilation there – as long as you are working with closed mouth. Better find a good rhythm – not too fast. I’m working with kettlebells myself, since my days in Russian airforces in the mid-eighties where I lerned the techniques. That time I used exhalation through pursed lips. Now after awhile with breathing optimization I find it better for both building up resilience and concentration.