Watch the Power Chords! online guitar lesson by Angus Clark from Essentials: Rock Rhythm Unplugged
When there's a riff that identifies the song, its usually fine to play the riff. If you're at liberty to do without it and start on the vocal, that's an option you can pursue, but very often the riff is what identifies the song, so it’s a safer bet to play it.
Similar to the last example, the right hand is aggressive and we're using left hand muting to get a percussive effect off the strings. So, consistency is key.
The challenge here is to avoid playing the thirds of any of the chords, and to give the arrangement some dynamics and range while keeping the feel all rocking, all the time.