I've been using Visio for years to do a variety of software related diagramming tasks (and occasional home improvement projects), it really seems to be the only credible product in the niche between graphic programs like Illustrator, presentation tools like PowerPoint, and full fledged CAD or CASE systems. Anyhoo after a long break from using any Windows apps at all I just started using Visio 2010 for some database design work and was baffled/annoyed by the fact that it randomly adjusts the zoom level to something other than what I set it at, e.g. on my main monitor if I attempt to set it to 100% it changes the value to 104%. Thinking it had to be a bug I discovered that this is actually intended behavior, and there is no trivial work-around. Instead the only "easy" option is to create a macro that executes this VBA command
ThisDocument.ZoomBehavior = visZoomVisioExact
which you'll then have to run by hand every time you open the file (any if you want this behavior to be global you'll have to add the macro to every type of file). If you want this to happen automatically you're actually invited to create your own COM add-in. Just so you can zoom. Seriously Microsoft? Think the guys at The Omni Group would ever suggest that to a user?