Rhubarb leaves contain poisonous substances, including oxalic acid which is a nephrotoxic and corrosive acid that is present in many plants. The LD50 (median lethal dose) for pure oxalic acid in rats is about 375 mg/kg body weight, or about 25 grams for a 65 kg (~140 lb) human. (Other sources give a much higher oral LDLo (lowest published lethal dose) of 600 mg/kg.) While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%, so a rather unlikely 5 kg of the extremely sour leaves would have to be consumed to reach an LD50 of oxalic acid. Cooking the leaves with soda can make them more poisonous by producing soluble oxalates. However, the leaves are believed to contain also an additional, unidentified toxin, which might be an anthraquinone glycoside (also known as senna glycosides).
In the petioles, the amount of oxalic acid is much lower, only about 2-2.5% of the total acidity which is dominated by malic acid. This means that the raw stalks may not be hazardous, although the tart taste of raw stalks is so strong as to be unpalatable to many.
ref: Rhubarb - Wikipedia, the free encyclopedia
under the heading "Toxicity"
Quite often, the old lore is just lore. I can't find anything about not eating rhubarb picked in any month with the letter 'r'. The explanation sounds very dubious to me. I've heard the "months that contain the letter 'r'" on things such as when to eat rabbits, but I've violated that many times. My first question when I hear it in this context would be around which hemisphere or growing clime that would be, and, of course, I would think it more likely that the toxin is produced in the leaves, but I probably won't search deeply enough to find those details.
The symptoms I read for oxalic acid toxicity from rhubarb are kidney disorders, convulsions and coma. Ref: List of poisonous plants - Wikipedia, the free encyclopedia
Rhubarb stalks are one of the most popular ingredients for cooking across the globe.
For what it's worth, many things you eat contain oxalic acid including potatoes and peas. I'm sure there are precautions to take, and it doesn't seem to me that much rhubarb will be used in a rhubarb-strawberry wine, but it's worth checking out. I wouldn't consider this board to be a good source on it, and that includes what I just posted about it.