I made Rice Krispies treats for the first time 2 weeks ago. I checked on the Kellogg's website (I'm not sure if I can post a link, but I'm copying the text and pasting them below):
For best results, use fresh marshmallows.
-1 jar (7 oz.) marshmallow crème can be substituted for marshmallows.
-Diet, reduced calorie or tub margarine is not recommended.
-Store no more than two days at room temperature in airtight container.
-To freeze, place in layers separated by wax paper in airtight container. Freeze for up to 6 weeks. Let stand at room temperature for 15 minutes before serving.
I had to make half of them in advance, so I froze the first batch (by following the instructions). I let them thaw 15 minutes before serving, and took a bite. I then compared it to a freshly made one -- they taste equally great to me. However, it is my first time to make these guys.