Bypass capacitor in combination with ground plane
You don't need a separate trace, but consider how the current will travel between the GND pin of the capacitor and the GND pin of the IC. 

When the current doesn't have to travel as far, the impedance (most importantly the inductance) will be smaller, which is exactly what you want for a bypass capacitor.

The same goes for VCC: Arrange the PCB layout so that the current has the shortest possible path between capacitor VCC and IC VCC.

Also these traces should not be too thin. Therefore, using the ground plane to connect between capacitor GND and IC GND is usually a good thing. 

Of course, if you break the ground plane by running some other trace between your two GND points, the current will have to make a detour around that other trace, which is not good. If it is necessary for some reason to break the ground plane in this way, you might want to run a regular trace between your GND points on a different layer as well. 

It's all about keeping the impedance between the capacitor and the IC as low as you can.