The problem occurs, when you are using strings, not when you use
the Header class directly.
The reason for this behaviour is the way the Generator creates
Header instances from strings: It initializes the class with
continuation_ws set to '\t' instead of the default ' ' 
(email/generator.py in method _write_headers).

>>> from email.MIMEText import MIMEText
>>> msg = MIMEText("Just a test.")
>>> msg["Subject"] = "foo "*22
>>> print msg.as_string()
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo
        foo foo foo foo foo 

Just a test.
>>> 

Tab as the whitespace character is no problem for most email clients. But
i.e. Outlook removes it.

When using the Header class you get the following

>>> from email.MIMEText import MIMEText
>>> from email.header import Header
>>> msg = MIMEText("Just a test.")
>>> msg["Subject"] = Header("foo "*22)
>>> print msg.as_string()
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo foo
 foo foo foo 

Just a test.
>>> 

The last message shows up in Outlook correctly.