john!
Smash Hero
this question comes up a lot. peach's fair and nair both have a 4 frame advantage on shield. dsmash hits on frame 5 and roll is invincible on frame 2. so if you do it frame perfectly, buffered roll will come out, but they will be hit on frame 1. otherwise, they can roll safely. frame perfect execution is really hard though, which is why peach players do mixups with slap and grab (slap comes out on frame 2, and grab keeps them from hiding in shield).
source: http://www.smashboards.com/showthread.php?t=287033
edit: whoops my momory is bad, it's spot dodge that is invincible on frame 2, peach's roll at least is invincible on frame 4, which gives you a larger window. some characters do have spot dodges that go through the whole dsmash, but i forget the list. this seems to indicate that fc to grab on shield works only when frame perfect, but buffered spot dodge can ruin that strategy.
source: http://www.smashboards.com/showthread.php?t=287033
edit: whoops my momory is bad, it's spot dodge that is invincible on frame 2, peach's roll at least is invincible on frame 4, which gives you a larger window. some characters do have spot dodges that go through the whole dsmash, but i forget the list. this seems to indicate that fc to grab on shield works only when frame perfect, but buffered spot dodge can ruin that strategy.