From my experience (on 100 mg of sertraline a day) an SSRI regimen will attenuate and dull the psychedelic experience. An approximate doubling of the dosage will produce equivalent, but clearly different, effects. The continuity and beauty of thought patterns will not reach the same heights, and visuals will be very bland and colourless. Instead there will be a great confusion, apathy and insecurity, with potential for actual delusions and hallucinations. These experiences have been with 4-HO-MET, which is quite similar to mushrooms. I should note that I was quite drunk both times (otherwise I wouldn't have bothered to ingest a psychedelic during my SSRI treatment), which could account for the deviations described above. The second time I had a very uncomfortable experience with very little sensory psychedelia but plenty of manic and paranoid delusions that urged me to seek medical attention.
In short: I don't recommend tripping while on SSRI as it will be a waste of good material. However, I've also ingested alcohol simultaneosly which could account for some of the discrepancies.
Also, bleh123 is very sound in his advice to be even more cautious with the phenethylamines.