This year I'd expect the PCT in the northern Oregon and Washington to be under considerable snow through mid summer. I live in Washington and Interstate 90 is on its second day of closures because of all the snow. Crystal Mountain, which is adjacent to Mt. Rainier, already has surpassed 80 percent of its annual snowfall average. At Timberline ski resort at Mt. Hood, Oregon, the amount of snow is epic, with in excess of 480 inches since September.
March is still a rather snowy time of year in this area, and I would not be surprised to see us tramping through snow come August in many spots. But of course, conditions can and will change.
As far as the best and/or safest places to venture in March (Washington has already had nine avalanche related deaths this season), others could speak to this question. I generally snowshoe in March, but generally go to the safer areas as I am not as confident in my winter back country skills. I think this season has been a grim reminder of how easy it is to get over your head. Snowshoeing around Timberline probably is your best bet. Another good place from what I've read is the Frog Lake sno-park. You follow the PCT north and decend to Lower Twin Lake.
But surely others could provide a better picture of the situation. You might also try portlandhikers.org. They schedule a lot of snowshoe trips and the people there could provide you with some local PCT recommendations.