I believe the software should add wide spaces after periods correctly, and not the user. Two reasons: first, two spaces is too big a separation between sentences. Second, it's really hard to maintain consistency (or ensure that there's consistency) in the use of double spacing without the help of software. It's easy to miss when reviewing.

Let the software do this menial task. That's what computers are for, aren't they?

A wide space after a period is only sensible when the period ends a sentence.

It would seem sensible to me that a word processor should treat a double-space as a hint that the preceding period ended a sentence, and a single-space as a (weaker) hint that it did not.

As far as the software is able to do this correctly ... it doesn't.

(Other comments already cover the rest that I would have said.) :-)