If some developers still use a 32 bit 'Windows XP' and others use a '64 bit Windows 7', would it be advisable for the entire team to work with 32 bit development tools even if their OS is 64 bit? Are there any problems that could happen if uniformity is not followed?
You have choices:
There are problems, but these are quite temporary - in that you might have a problem with someone's code where they made assumptions that are not true on a different architecture. This is not such a bad thing, as its better to find these problems before your customer does. So all in all, standardise on a dev environment if it help you deploy new developers quicker, but otherwise concentrate on the end-product quality, no matter what the developer used to build it.
There's far less probability of issues if you all use the same toolchain.
There may not ever be a problem, but can you soak the time spent investigating why "it works on my machine" for our 64bit friends, and not with your 32bit ones if there is? It's usually the best option to go with lowest common denominator.
Behaviour does sometime differ from platform to platform, particularly for optimised release builds.
Again, very unlikely that there will be a problem, but always a good idea to standardise things.