I had this with my last birth - was told beforehand that they wanted me to have a cannula put in when I go into the hospital, as a 'just in case' thing, because I had previous PPH. TBH it didn't bother me - the midwive did a great job putting it in, it hardly hurt and I barely noticed it was there. BUT in the end I didn't need it - no PPH (which I firmly believe now was due to the fact that I was not put on a synto drip - the induction drug increases your chances of PPH).

Anyway, no you don't have to have it put in - but be prepared to be firm about it, because they will most definitely be against the idea, and put it in writing or sign a waiver or whatever so that they have it documented that you refused.