Ride quality and stability of the car may be affected. On a different note, some aftermarket manufacturers are supplying replacement struts as complete assemblies, including a new spring and accessory components. In the short term, these assemblies might be more expensive, but are much less labor-intensive to install. - See more at: http://www.knowyourparts.com/product-category/servicing-macpherson-struts/#sthash.8NNkFMZR.dpuf
Yes Bazile, the top bearing on the strut could well be causing the creaking noise, however without knowing to what extent it is worn I wouldn't hazard a guess at what damage it could course. If were me I would change them both ( left and right ).
Get somone to checkthe " Bounce" on the offending corner, it should return once on "press down" Get a friend etc to spray WD40 In the direction on the inner top of the 2 front strutts, I would think if the bearing was going it would be a continune groan.
Is it a Vectra C by any chance? They are buggers for getting through bearings at the top of the strut. They just get noisier and won't damage anything else, and cost about £40 in labour to change at any independent garage.