Hi Katie---"Hiking" means different things to different people. For some, it means riding a lift up and talking a walk on a level path that has been cleared of snow. To me, it means a walk on a rugged trail with lots of climbing and descent, in places where one would need crampons and ice ax (or snowshoes) in May because of the lingering snow. For that kind of hiking, I wouldn't consider going before June.
If you are in the latter group, the Berner Oberland may not work for you in May. The Männlichen cablecar at Wengen, which accesses some great hiking terrain, does not open until 28 May. And although the Schilthorn cable car will be open in early May, you couldn't hike from there---the route down would still be mostly snow-covered and treacherous. (My kids encountered snow up there in September one year.)
You might try asking your question on the Switzerland forum on Tripadvisor. There are people there who live in Switzerland, and can advise you on the best places to hike at lower elevation, where snow won't be a concern. Or maybe they will say this is a "low snow" year, and much of the high country will be open.
I do know of people who have gone to the Berner Oberland in May and loved it. They were mostly doing lower-elevation walks, riding boats, and sightseeing, not doing serious hiking. Lauterbrunnen is a good choice if you want to do that; the valley walk and Trummelbach Falls should be fine, and you could do the cablecar "loop" up to Mürren, walking down to Gimmelwald, riding the cablecar down to Stechelberg (very exciting cablecar ride), and walking back to Lauterbrunnen from there.
I have also seen photos of Gimmelwald in May on the web and it was very green and looked nice---so if your hiking is at that elevation and lower you might be OK. We've mostly hiked up higher (above Mürren and above Wengen) so that is the personal experience behind my answer.
