I clamp the vacuum hose first and allow the resin to keep flowing for 20-30 seconds, but it depends a bit on how your infusion played out. Sometimes you may get a bit of resin pooling in the first part of the stack for whatever reason, and in this case you would want to clamp the resin feed and allow the bag pressure to equalise.
You'll get a feel for this though. For the most part, clamping the vacuum hose and letting a touch more resin flow through will ensure a perfect cosmetic finish.