I've had this discussion before and i'm confident that 5-HTP will not cause SS with MDMA. Firstly, 5-HTP takes a few hours to completely metabolize of which the MDMA-induced serotonin will be less. Secondly, 5-HTP does not, at least to my knowledge, act on any of the serotonergic pathways of release. Thus, quite simply taking 5-HTP won't cause serotonin to be released into the synapse. SS occurs as a result of an extreme and severe excessive amount serotonin residing in the synapse. Thirdly, if 5-HTP & MDMA could cause SS, then consuming a diet extremely rich in L-Tryptophan, immediately before ingesting MDMA, would also be dangerous - right!? like Rickolas pointed out: L-Tryptophan - 5-HTP - Serotonin. It is a natural building block in the pathways of Serotonin production - NOT a serotonin releasing agent (Like MDMA). I see the rationale that increased 5-HTP = Serotonin available = More serotonin released as a result of MDMA = Increased risk of SS. However, if this truely was a risk worth discussing, then eating foods high in L-tryptophan, while rolling, would also be dangerous due to the natural conversion into serotonin, via the pathway mentioned, then released into the synapse.
Long story short: I think its a myth thrown around without any true scientific underpinning to back it up. In reality, the risk of SS is so small I don't even know why people bring it up. We might as well start worrying about the ridiculously small chance of spontaneously combusting at any point in our life as well.