I haven't used the exact instrument you mention. However, my experience with platelets is that because they are highly reactive (and can be stimulated by virtually any material to a certain extent), I would change membrane every time -- especially if this is not an expensive item to change. Better safe than regretting bad data.

Perhaps more directly, you could simply prove repeatability of your assay: with the same sample, run several assays with the same membrane, and see if you get the same or different answers. If you do see different answers, try this again changing the membrane each time. I would also recommend trying this on a few different blood samples -- basing the 'proof' on merely one blood sample might be misleading.

Thanks alot sir for precious suggestions!