While your in school, it's simple interest - so if you take out $10k, and let's say that interest accumulates at a rate of 10% on a monthly basis (not necessarily true to life, but for our purposes), after the first month you'll have $11,000. The next month, interest will still only be calculated from the principal ($10k) instead of the total amount ($11,000).
After you graduate, future interest will be calculated in a compounding fashion - that is, if you had $10k to start, and accumulated $1000 in the first month after the grace period, the next month's interest will be calculated off of the $11,00 instead of just the original principal of $10,000. You can see how this quickly gets ugly.
In your case, I think you might be better served by avoiding taking out loans the first year entirely. Would this wipe out your funds? Or would you have any left to take a chunk out of your loan package the following year?
In any case, although the 7.9% GradPLUS rate is pretty bad, 6.8% isn't so hot either. If you can delay taking out any loans in the first year, you could save yourself a good bit in the future.
As I was thinking through this issues for myself I while back, I made an excel worksheet that allowed me to figure out some rough estimate for different scenarios. It's not perfect, and assumes no repayment during residency (which will hopefully not be the case!), but it's good for a ballpark estimate. Take a look at it and plug in your numbers for yourself. It's ugly, since I just intended to use it on my own, but it works well enough.
First, when you open it up, you'll notice the bolded numbers on the left. These represent your Stafford and GradPLUS loan amounts each year for all four years. Obviously, in reality, these numbers might change a little, but again, it works. Enter in your projected first year loan amount here (don't count your extra funds, just enter in what you would have to take out to live on loans, pay tuition on loans, etc.). Then, on row on the top represents your twice-a-year disbursement of the loans you just entered in. To see how using your funds to pay for the entire first year would work out, enter in "0" into the Stafford and GradPLUS rows under Fall 2013 and Spring 2013. Now look at your "Total Balance After School." Remember that number. Now, use the undo function to restore the worksheet back to how it was after you had just entered your projected yearly loan burden. Enter in "0" in ONLY the GradPLUS rows for Fall 2013, Spring 2014, Fall 2014, and Spring 2015 - this simulates using your funds to pay for GradPLUS loans over the first two years. Then you can decide which option will save you more money.