In An Essay on Criticism Alexander Pope writes: 'The sound must seem an echo to the sense.'  He continues:
'When Ajax strives some rock's vast weight to throw,
The line too labours, and the words move slow:
Not so, when swift Camilla scours the plain,
Flies o'er the unbending corn, and skims along the main.'
It's still great advice, and I endeavour to follow it.  More than that, I find that it isn't only the words that change tempo, but the speed at which I write them.  If I'm writing something sad, I write more slowly.  If it's a passage of action, I speed up.  When it's humour or verse, I tend to pause with the commas–timing's so important when it comes to punchlines.  
Perhaps the moral is–if I want to get more work done, I should stick to action and avoid getting sentimental!