Thirty-four years ago in high-school typing class, I learned to hit the space bar twice at the end of each sentence. This was good advice, because my instrument was an Underwood typewriter (not as old as this one, but almost).
The practice of using two spaces between sentences and indented first lines to begin the new paragraph are merely remnants of days when attorneys had only typewriters at their disposal and were forced to use a monospaced font. Using a monospaced font requires two spaces between sentences in order to provide enough visual cueing through width. But using two spaces with proportionally spaced fonts will create extra gaps, which will cause the reader to experience a greater fixation pause between sentences as her eye searches for the next phrase.