With Shimano you shift to a larger cog or chainring by pushing the entire brake lever inward. Derailleur cable tension resists the sideways motion, so it takes a deliberate push to move the brake lever sideways. I'm not really a Shimano user, so I can't say how "loose" the lever will feel if there's no tension.
With SRAM, you use the lever behind the brake lever for both up and down shifts. SRAM brake levers can't move sideways, so the lack of derailleur cable tension can't have any effect on the brake lever itself. This is also true (well, the no sideways brake lever movement part) of Campagnolo and Microshift.