I have been using my credit card or cash to pay for my parents groceries and other necessities and they reimburse at the end of the month.

Jimmie, same here regarding keeping track of receipts and checks/credit cards. Everything is kept in a large 3-ring binder.

If there is a large bill where I need to pay from my own checking account [for whatever reason] and my parents would later reimburse me, I would take that bill and make a copy of it with my check.... and then again with my parent's reimbursement.

That is how I am doing it. I use Mom's card and checks for her groceries, incontinence products, really, any of her expenses. Once in a while I will buy something with my money and she will reimburse me and I just factor that if those amounts are large enough to catch Medicaid's eye, that I will just have to pay it back (even though it was her expense)

I don't know the official answer but my suggestion is to keep the finances clean and separate. I have seen the term "co-mingling of funds" and apparently it is to be avoided.