If you aren't an illustrator, doing dummies is useful for you in your creation (to make sure you have enough events in your story to illustrate a full book and maybe to examine pacing). Of course you don't want the text weirdly spaced across the page, but page breaks are more than simply counting up an evenly distributed number of words across the pages. Page breaks indicate change, new scenarios, places to add surprise or a twist or punchline. Those things are determined by plot and not just word count. So making a dummy can help a writer see thin parts of the story or places that drag, and they can revise accordingly. As you said, it can feel different when you are reading it off the computer and when you are reading it with actual page turns from a dummy. So as far as in writing, if it helps, go for it!
In submissions, though, unless you are the illustrator and are submitting illustrations, I would not mention or submit the dummy. Agents are used to seeing straight text and know what to do with it. They may sometimes suggest writers create a dummy for the purpose of writing/editing, but it's not something they need to see in a submission. An illustrator will make their own page breaks (which may or may not fall on the same lines of your practice dummy), so it's not like it would be used later on. The important point in your submission is that you came up with a strong story--no matter what you did to get there.