Do you need a doctors letter when boarding with Insulin Pens and Needles?

No. But specifying a country would help. Or the fact you're presumably talking about air travel.

Have flown dozens of times with supplies packed in a range of ways, including just a few things (test strips, syringes, insulin vial) loose inside my toiletry bag, and also gallon ziploc bags stuffed full of pump and CGM supplies. This is mostly domestic USA flying, but some international in Europe and SE Asia. Never once even had a single question about it.

No.

I got in touch with my Endo and she wrote me a letter stating the insulin and pump supplies I need. She also wrote me a generic insulin prescription in case an emergency happens and I need to pick some up from a pharmacy.

No, they only ask if you have water or juice with you.

I don’t believe you need them but I always make sure I get a doctor’s letter to cover me as I’ve had some issues with an EpiPen. Or even if you don’t have a doctor’s letter, a photocopy of your prescription can be a good thing to take and shouldn’t cost you anything.

No, but it's handy in case you end up needing medical care to have a list of your diagnoses and medications ready from your pharmacy/doctor to hand over so they can provide you the care they need.

Happened to me last March, I ended up hospitalized on a trip in a foreign country and it was a good resource to have,.

I flew US from Canada last year, no note needed. Might have changed though.

No, but I always make sure to bring the prescription label off the boxes just in case. I have a TSA approved Diabetes cooler pack that has an exterior pocket for these, so I just unzip it and toss it in a bin.

TSA generally just looks at it, shrugs and hands it back.

Maybe?

E.g. the UK's CAA says:

Take supporting information such as a letter from your doctor or your prescription with you. You may need to show this to the security staff at the airport.

But in practice, I have been asked to present any such evidence in the 15 years since I was diagnosed.

I took one just in case (uk to Greece) as I have insulin pens and needles but no one checked. Always best to air on the side of caution

I never had anyone even comment at all. But, I have carried paper evidence of prescriptions for everything that needs one just in case.

Might be handy to have something from a doctor in case you end up needing urgency replacement supplies wherever you're going, though. I figure the prescriptions should double for those purposes, though.

Never!

I've carried a drs note since 1996 but have never had to show it in Canada, Mexico, England, and a few places in Europe.

You don't need one. I am traveling to Asia in the beginning of March and my Endo gave me a note anyway just in case it's an issue. Even she said "you most likely won't need it but I'll make you one anyway just in case"