Only if the ads are a fixed length and always in the same place for each playback of the same video.
Inserting ads of various lengths in varying places throughout the video will alter all the time stamps for every playback.
The 5th minute of the video might happen 5min after starting playback, or it could be 5min+a 2min ad break after starting. This could change from playback to playback; so basing ad/sponsor blocking on timestamps becomes entirely useless.