Re matrix patches - Dr Currie confirmed on here and in the magazine that it's fine to cut patches off-licence. Any effect along the adhesive edge is likely to be marginal but may possibly be affected by the length of the cut area in relation to the size of the patch (?) - but we're getting into minutiae here!
I also think it depends whether you are retaining the residual part of the patch for later. There is likely to be no important effect from cutting a bit off the patch and sticking it on straight away, although your own cut edge may not be as clean as the one supplied. However I really wouldn't worry at all. If you have patches available then get them asap! They may not be available for long!
If you are keeping half for later (eg 3 days) I would suggest that once the air has got to the patch then the cut edge (that is left behind) may absorb a little moisture making it slightly less sticky - but probably won't affect the whole patch. Just keep having a feel to make sure it's stuck down! I regularly cut patches and have not noticed anything. The only time it's been an issue has been when I've been on holiday and cut up eg 2 weeks worth ( cut down 75 to make 62.5) - when I came to try to stick the patches on that I had cut a couple of weeks ago - I found it hard to get the backing but off ( this is with Estradot) because I think a little of the adhesive had oozed out (miniscule amount though) - it made no difference to how I felt though! Probably also depends on the ambient temperature.
I do keep the cut off pieces now I'm on Estraderm (cos they're large) in case of extreme shortage - in which case I really won't care about exact dose anyway (not that it is an exact science!)
My advice to anyone cutting patches is to cut no more than one patch at a time and use it up quickly and you should be fine

Hurdity x