I see some confusion going on here, the negative interest rate would be on deposits at the ECB made by private banks in order to stimulate other kinds of investments, private deposits will yield at whatever interest rate your bank applies. Close to 0 anyway in most cases.
The euro is on a negative trend over the dollar since a few days, so buying T Bills would have been a good idea and probably still is. I think that they'll do "whatever it takes" to push it close to 1.20 or so. Otherwise it will blow for the joy of bitcoiners all over the world

Half Europe is deflating, we will know in early june if the ECB is doing something against it, there's also some talking about a QE or some other sort of assets buying. European elections results may be crucial.
Definitely not the best time to hold euros
IMHO