I think it hinges on whether being told you have 'lifelong' issues is a good or a bad thing.

For me, I was self-help book addict and spent every hour of every day trying to improve myself, to no real success. Receiving my diagnosis was a huge weight of my shoulders because it allowed me to start to accept myself for who I am and not put to much effort and energy into masking or trying to 'cure' my autistic traits.

But I could also see how that same result would be upsetting for someone, to know that these traits will never go away no matter how hard you try, and having to come to terms with that.

Either way, knowing you have ASD will be beneficial because how to deal with ASD issues is very different from non-ASD issues.

For example, if you get overwhelmed and stressed in busy/loud/bright places like a supermarket, for ASD people this is to develop a routine and wear noise-cancelling headphones. For non-ASD (maybe just social anxiety) this could be CBT or exposure therapy, going out more and more until you become less uncomfortable. But that same advice could be catastrophic to someone with autism.