The University of Minnesota does not have a specific publication about this, so I went to other universities (see below). Here's what I learned:
If rhubarb has suffered apparent frost damage, it is recommended you not eat it as oxalic acid from the leaves may be transferred to the leaf petioles (the stems we eat) and is considered toxic. This is particularly true if the stem has been damaged by freezing temperatures and / or is mushy. When in doubt, throw it out.
When you pick rhubarb (not frost damaged), cut the leaves off the stems immediately and compost them (Yes, you can compost them because the oxalic acid and soluable oxalates are not readily absorbed by the roots of plants. Compost containing decomposed rhubarb leaves can be safely worked into the soil of vegetable gardens. Source: Jauron, Richard, Iowa State University Horticulture & Home Pest News, May 2, 1997 issue, p. 57).
You can then use the rhubarb as usual or chop and freeze it. Freezing the stems only does not produce a toxic effect as it is the leaves, not the stems, that contain oxalic acid.
Here are some good resources about rhubarb and toxicity:
General information about growing rhubarb: