In the UK it's pretty much always cooked because it's pretty much always H3 so it needs an acid (citric, vit C) and it's heated to basically speed up the reaction. There is definitely an element of bacterial killing as well when my fiend does it but not too much a few bubbles and that's it, apparently too much heat can break it down.
If it's proper good gear like relatively pure H4 with a proper soluble cut then i have heard people in Australia for example use plastic spoons and simply mix it, no heat obviosuly. I have never seen proper medical diamorphine in ampules but i saw a story about it here and as far as i could tell the amuples where in fact powder diamorphine and sterile water was supplied then it is reconstitued, so i imagine (can't say for sure) it would dissolve on contact lovely clean, sterile known concentration. Unfortuntely that it not the case for the vast majority of people and they have to contend with street heroin
and possible containimation etc.
I suppose it's down to who the person learns from when they first encounter IV use, this can often lead to bad technique that is carried on until they see another person do it properly. So many users out there don't really know anything other than what they have seen and been told, you talk about H3 and H4 to the averages user and they'd say huh?