Depends on the mag, mostly. Good example is the two mags that came with my Shield. When the sixth round is inserted in the 6 round mag, there is room for about another half a round. In the 7 round mag the seventh round meets didtinctly added resistance during the last part of insertion. I interpret that increased resistance as extra stress on the spring. I don't want the spring under that stress for long periods, so I don't replace that seventh round after chambering it.

Yeah I know what you're thinking - I'm a wuss so I think the spring is a wuss too. Hah.