It very much depends what materials you intend to steam clean. I had a customer bring a car to me a couple of years ago now (vauxhall omega from memory) who had botched an attempt at steam cleaning the upholstery. What had happened was the sheer heat of the steam had in effect flattened the nylon fibre down on the velour upholstery, almost making it look bald in places, if you can imagine the velour is millions of upright short nylon fibres, and in big patches they had become horizontal to the base material.

Needless to say, by that point the damage had been done, and there was nowt i could do to resurect the trim.

You'd be better of with a decent interior cleaner and a good wet/dry vac. Take your time, dont overwet the cloth, or scrub too hard (cos that can damage the fibres as well), and dry naturally, you should be ok mate.