It’s considered a best practice to not configure the ‘bandwidth’ statement on physical router interfaces. This is because the routing protocol will throttle updates propagated through this interface. The one case where you’ll need to consider configuring the bandwidth statement is on Tunnel interfaces. This is because the default bandwidth of a Tunnel interface is 9.6kbps.
Reference: IOS Interface Configuration Guide
security without obscurity 
What about for serial interfaces when the actual circuit would be something like 64000 but the default bandwidth for serial interfaces is 1544000?
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Newer IOS versions bump the default tunnel bandwidth up to 100 kbps.
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