Yep, don't risk it yourself
First time I ever ordered a carpet, I was 20 and had just bought my first flat wtih (now) DH. I measured it to the last mm, not knowing I had to allow extra (bedroom carpet)
When teh bloke turned up to firt it, it was a bit short and I burst into dramatic tears! I sobbed "its my first carpet" and he said he'd tell them the cutters made a mistake, so I got a properly fitted carpet