When we went at the end of August with our double stroller, we left everything but valuables (cell phones, money, credit cards, room keys, etc.) in the stroller and it was never touched. Occasionally our stroller got reparked by CMs, but otherwise no problems.
That is what we always did. Always left the diaper bag, ponchos or jackets and souvenirs etc.
Don't fool yourself, there are thieves at WDW! Don't leave anything in your stroller you wouldn't want to try and replace.
Not so long ago someone posted of seeing their stroller walk off into the parking lot. When security caught up to the thieves the had a trunk full of stolen strollers.
