It's never a good time to buy a MacBook Pro, there's always a newer, better one coming out.
Seriously, though --it all depends on where you are starting from (and what you need).
The May 2015 release is hardly a "silent" update. Upgrading from a 2012 15" rMBP got me Thunderbolt 2, 4x faster flash storage, faster wi-fi, faster discrete graphics (AMD), far superior integrated graphics (Iris Pro), significantly improved battery life, and last, but not least, the awesome Force touch trackpad.
However, owners of the late 2013 and mid 2014 models would do better waiting for the next edition (presumably with Skylake processors).
Want a hard-and-fast-rule? Always buy
just after a new model release, but NEVER
just before a new model release. (MacRumors Buyer's Guide really helps with this.)