Of course, there is only one answer to whether tabs should be allowed in a source file. The utility of one is just so obvious I'll simply avoid commenting on it. :-)
If you must wander from sources written with one convention to those written in another, you should definitely set Visual Studio (or whatever editor you use) to show whitespace as visible. You can change this in VS by going to Edit/Advanced/View White Space.
I also change the default color for visible whitespace to silver. Silver on a white background is unobtrusive, yet noticeable when there is inconsistency.
(Next time, I'll go into whether Big Endian or Little Endian byte ordering is better. :-)