Which boat?

On the older designs with strongly curved decks (Chesapeakes and West River being the ones I've built) you glass, then cut. The hatch covers are usually separate pieces of wood with formers bonded to them to shape them.

On newer designs like the Wood Ducks, you cut, then glass. The cutout is re-used as the hatch cover. That boat has a flat deck with a flat cover, so deformity is not such a problem.

Not sure about stripper decks, haven't built one of those.

It really depends on the boat type and the deck curvature.

Laszlo

   I always glass then cut. Less chance of the thing coming apart (if it's a stripper).

Dan

   Sorry would help if I told you it's a Mill Creek 13 so front a bit curvy but the rear is flat. 
reason for this is I'm staining so seeing the outline is easier on bare okoume

also seems like a waste of time and material

I have hatches already made and was hoping to use the cutouts for color and finish experimenting before staining glassing and varnishing the whole thing I'd like to make certain of what it's going to look like and make adjustments if necessary. 
thanks PP

   Per instructions, on my Shearwater Sport (flat deck) I did the glass then cut method  On the Shearwater I did flush hatch/invisible hold-downs.  On my subsequent Chess 17 I wanted to accomplish the same thing, and I wanted the deck grain to match (not use a separate piece of plywood for the lid), so I did the glass then cut routine again.  I wondered how much "snap back" - if any- there would be to the cut-out piece. The answer was about 50% of the curvature.  After building a semi-custom flush-hatch rim/lip, when installing the strongbacks to get the hatch lid curvature back to matching the hull, some finicky art was required - the as-built curvature of the hull wasn't a perfect match for the as-cut kit hatch strongbacks - probably nothing that would ever have been noticed with the "sit-on-top" standard Chess hatches. But it was surprising how being just a bit off in the curvature was going to mess with the ability of the hatch to seal and make things look not quite right. Like on the car lot when the door and car side panels don't match up perfectly - either it's a low quality car, or has had an accident. So I had to carfully modify curvature of the kit-stock strongbacks by putting a strait edge longitudinally on the deck over the hatch, checking for a uniform gap as I carefully held the strongback in the position where it would end up when glued to the hatch lid.   Anyway, I even had to heat-gun one strongback off after assemply on one hatch, then re-shape the curvature a bit and then re-glue.

If I had to do it all over again I think I would still have used the cut-out piece of the deck to make the flush / invisible hold down hatch lid, but would have cut it out PRIOR to galssing, then built the hatch rim and got the hatch lid all made and fitted perfectly, then glassed the deck and the hatch lid as separate pieces, with the standard tape/plastic arrangment to keep epoxy from messing up the hatch rim while glassing the hull.