Usually in May the ID is still a good plan. Last year late May/early June was when ID route was out and the DC was coming in. Check out http://mountrainierclimbing.blogspot.com/ for info and reports from the previous seasons. If you plan on staying at Muir or Ingraham flats there will be plenty of people to get beta from on the condition of the route. You won't need slowshoes for the approach in May. Also you can find some good information on these forums, there are plenty of great climbers here to get advice from. If you have anymore questions on the route just send me a PM.
Cheers!