I have done a lot of presentations over the years, and to the best of my recollection, every single one has been done using a Linux machine, and this has mostly been a non issue.

I think a better adage might be "get there early so you have time to deal with technical issues, bring any potentially required adapters, and have your presentation on a thumb-drive in case you need to borrow someone else's machine to present in a pinch." I say, "on a thumb drive" as opposed to "in Dropbox" or whatever, because you may rarely find yourself in a position of not having net access in the location of the talk.