I over thought this for SOOO long. I'm not a real musician and tons of people know tons more than me but I think if you know the song well just strum what is naturally and you'll have it. I found if I sing with it that keeps my timing for me (my voice is AWFUL but I play for fun in my own home!). As my cousin told me (a music major and guitar player) even the people that wrote the song play it different every time. If you know the progressions from the tabs the time to change just takes care of itself. The one thing I found is often though the tabs are just wrong...you'll play a chord and hear there is no WAY it's right.

Just my two cents....