Everyone should be advised that according to

Factory Technical Bulletin #9305 of April 15, 1993 "Retrofitting of 18" wheels on Porsche vehicles which do no have 18" wheels as original equipment is NOT ALLOWED (bold and underlined!).

The factory says that endurance tests have shown that equipping certain Porsche vehicles with 18" wheels in an application not already approved by Porsche Cars may result in loosened bolts and/or threaded connections on the front wheel carriers. This, in turn, may cause fractures in these components. Fractures may also occur in the front strut mounts and in the body in the area of the rear suspension as a result of the installation of 18" wheels.

Such damage can occur without warning.

This bulletin applied to ALL models of cars.

Also of interest is Technical Bulletin #9303, September 28, 1993, which deals with the fitting of 17" wheels and tyres to 4-cyl models from 1987 onward, 944, 944S, 944T (all with M30 option), Turbo S 1988, Turbo 1989, and S2 from 1989.

Fitting of 17" wheels on vehicles previously equipped with 16" wheels is allowed, however you should also replace the caster adjustment parts so that the alignment can be adjusted for the proper caster angle of 3 deg 15-45'. The parts needed are:

Rear Mount    951 341 023 00,    2 each 
Eccentric        951 341 431 02,    2 each 
Bolts,                                                4 each

The 951-341-023-00 part has been superseded by the -01 part. There is also a 968 Club Sport part available, 951-341-023-30. The upgraded 951 part is also used on 968. 
There is a huge difference between it and the stock part. 

The stock mount has a LOT of rubber and only a thin metal cover around it, like that used on a antiroll bar. The upgraded part has just a thin rubber insert with a substantial metal piece around it. 

944 turbo Caster mount

The eccentric bolt is also used as standard on the 968 and I'm told it is serrated to hold the caster setting better.

944 turbo eccentric

There is a  certificate from Porsche (Nov, 15th 1996) which states what additional mods have to be done to the car when upgrading to wider rims/tyres. This certificate lists all (in Germany) allowed
tyre mods for all 944 models.

When upgrading to 7.5x17,offset 65mm (225/45) and 9x17,offset 55mm (255/40)
you also have to install (among other things you have to check):
# 951.341.023.01 (rubber bearing)
# 951.341.431.02 (eccentric bolt)
The listed parts are to be installed at the front suspension, not the rear.

When upgrading to 8x17,offset 52mm (225/45) and 9.5x17,offset 68mm (255/40)
you have to install the above parts and additionally a 17mm spacer for the rear rims (#951.331.611.02)