I'm working on a PCB design with 5 total power rails for components with various preferences, and I'm wondering if it's a good idea to provide small 'island' copper pours for the rails underneath their associated component pins?

I'm imagining the effect as in the attached diagram, with it providing additional local decoupling, but I'm not sure if this effect is more or less beneficial than keeping the power plane as intact as possible.

See the attached screenshot for an annotated example of what I mean.

 

Welcome to AAC!

I'm wondering if it's a good idea to provide small 'island' copper pours for the rails underneath their associated component pins?

There's no way to make a general statement about the effectiveness of doing that.

Deoupling isn't the only potential benefit. Critical signals with shielding from power/ground islands on the layer above or below can also benefit from reduced cross capacitance to other signals. A good routing strategy can also minimize cross cap.

 

if it's a good idea to provide small 'island' copper pours for the rails underneath their associated component pins?

In general I would say yes.
Try to group the components to make the islands as large as possible (but not if this significantly increases critical signal trace lengths).

 

I have used such islands on the specific instances where the copper pour will assist in heatsinking a power device.
For instance, the collector tab in a source follower.