I have had some really bad trips on acid (2 of them) 1 of whch was altering, as far as shrooms go I have had 1 trip that went very very bad due to unforseen circumstances otherwise would have been one of the best trips of my life and it altered me greatly.
The bad trips ive had on acid didnt alter me anything like the shroom's did, after my last bad acid trip I was kinda weird for a week or two but after my last bad shroom trip I was all fucked up for a good while 3 months or so.
I havent had a bad trip in almost 2 years now I think thats how long its been.
errr so I guess my point is that a bad trip on shrooms can be just as bad if not worse than a bad trip on acid all depends on dosage/enviorment/people with you/your mind set.
I recomend reading alot on tripping before doing it,
http://www.shroomery.org/ thats a good place to start.
If you prepare yourself right and make sure your enviorment/mood/mind set and most importantly the people your with are good then you should be fine. Just dont think about bad trips all the time if you go into it thinking "O man im gona have a bad trip" then you'll just be condemned.
im rambling high so ill stop talking now hopefully this made sense.