Aren't there MAOIs in some cacti?
Could at the least make effects and interactions unpredictable.. I don't see why you should take 5-HTP the day of, and before.
I also don't really believe that pre-loading works unless you would otherwise have serotonin deficiencies for some reason.
It doesn't seem like it's a rate limiting step for your serotonin levels other than that and there is proof that it works for these applications, just no scientific evidence afaik.
You are better off saving it for nothing else than for times when you think you are unusually low on serotonin, not to boost it.