Back to the topic!
As a VFR pilot you can elect to monitor the ATC Frequency (the fluffy clouds on ERC - by the way who outside of ATC knows where half these locations are anyway!).
Having decided to monitor the closest frequency to your track (because you are cruising at a highish level in Class E airspace), it might be reasonable to assume that IFR traffic in proximity will also be on that frequency.
Sorry - your assumption is flawed (some of the time anyway). ATC sectors still exist with fairly rigid frequency boundaries in both the vertical and lateral dimensions. IFR traffic could well be maintaining primary listening watch on a frequency which is not on your chart.
Keep a good look out!
Aus ATC