No it's not necessary if your spice is clean after the first pull. I re-x if there's any foreign particulate matter in my precipitation dish or if for some reason my xtals have degraded/are gooey (I have had nice crystals degrade due to not being stored in a cool enough place). Pure white crystals are a nice "parlor trick" so to speak and will undoubtedly enrapture your you and your friends. White crystals are pointless, imo, as I prefer dmt + dmt n-oxide and full range spice (jungle, jimjam, pick your terms) to pure white crystals. Especially when you are making changa, there's really no point to take all the time and lose whatever yield through re-x if your product doesnt truly need it.
I like to extract white crystals, then oxidize them yellow, or use limonene to extract fullrange, containing all the alks and some oils.
Now, there are people who you will encounter who will blather on and on about white crystals and claim that you don't know what you're doing if you don't extract white crystals. I have encountered these individuals at various parties and if dmt gets brought up, their first question is "But do you know how to make white crystals? I won't touch anything if it's not white. That means that you have chemicals in your dmt and don't know what you're doing or have a bad extraction method". These people are ignorant and have no clue what they're talking about. They've probably read a tek or two or heard people talking and decided that purity = white and anything that's not "pure" is toxic or garbage or a waste, and as long as the coloration is not from your basic solution, that is just wrong. There are a host of alkaloids and oils in mhrb that can have very nice and synergistic effects with dmt when smoked and prevent it from being white. White does equate to pure n,n dmt, but off-white does not equate to toxic chemicals. Non-white spice that is not brown or speckled is probably just laden with other alks or oils in addition to dmt.
peace
SB