Plugging rear ports of "bookshelf" speakers?

...and potentially altering its intended sonic signature. 

Hence why I prefer sealed designs. Easy to place them in my bedroom system- I put my ATC 7’s on cork and rubber feet directly on my 72” low dresser about a foot from the wall, and they image really well with no bloat in the bass.

Think of plugging a port as a tuning option.  The options lets achieve good bass in a variety of situations compared to a speaker that did not have a port.

@noromance is correct, you ARE altering it's sonic signature so that it sounds better in whatever location you place it in. 

Speakers aren't golden cows to be worshiped and rotate our lives around them.  If you need a speaker on a bookshelf and it sounds better plugged then do that. 

@erik_squires +1

Exactly. Speakers + Room = most of the acoustics. Optimize both and you optimist the situation.

@immatthewj 

If the speakers came with plugs (e.g. many KEF models) they are designed to be plugged so that the sound will be more balanced when you place them closer to the wall.  The KEF LS50 for example has three modes- open port, half plugged and fully plugged.  The manual specifies the best position from the wall for each mode.  More plugged = closer to wall.  

However many ported speakers are not designed to have them plugged.  Woofer suspensions are such that they can be designed for stiffness / wide open port, hybrid stiffness where they can be both ported and plugged, or very loose suspensions that require them to be sealed.  

First step is to consult your manual to see how close they recommend placing them.  You can always experiment by plugging the ports.  I tried it once with speakers not designed to be plugged and it ruined the whole sound and distorted the midrange.