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I think a lot of journals will expressly allow this in their copyright policy. I just poked around, and I see for example that the AMS allows this:

and even Elsevier allows this in its journals:

So you can probably even do this completely legally. I would just check the web site of the particular journal(s) where you are considering submitting articles. And journals will even allow you to modify the copyright agreement in many cases, so you could still ask about this if a journal doesn't specifically address the issue already.

Added later: my last comment was inspired by having read the article Do Mathematicians Get the Author Rights They Want?'' by Kristine K. Fowler in:

March 2012 issue of the AMS Notices (appearing in the left column of the issue web page)

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I think a lot of journals will expressly allow this in their copyright policy. I just poked around, and I see for example that the AMS allows this: