I usually use the Ministry of Defence Discount Partner Rate (unless the Government rate works out cheaper with breakfast thrown in).
The ihg Partner Voucher is exactly the same no matter which 'partner brand' one is staying under. However in all my stays in ihg hotels across Europe I have never been asked to show it.
Even when I have offered it to reception I get told it is 'not needed this time'. I just carry a blank one around now on the off chance someone may eventually ask for it, but I somehow doubt they will
Overall the partner rate always seems a few pounds cheaper than the ihg 'best available' rate, so it is worth having.
However there are a couple of ihg chain hotels that have all dates 'blanked out' and so are not available on Partner Rate.
BM