On alloys a powerful yet washer can strip the clearcoat/paint.
On wire wheels very high pressure can damage the spoke seals.
There has been an investigation by DEKRA which shows that high pressure washers can damage the sidewalls.
I cannot find the report on the DEKRA site but the link below has some more information;
http://www.tyresafe.org/media-centre/lat...age-your-tyres/
