One of the things that really ruins a display of tapers is when the candles are not standing straight in the holders. In addition to looking bad, a leaning taper candle could eventually start to drip, even if it is the dripless kind.
There are 3 main reasons that taper candles lean.
We will start with how to deal with the the crooked candle holder first because there are not that many options, however, there are a few things you can try. If the holder is a metal one, like in the image above, you might be able to bend the arms to make them straight. For holders that are sturdier or 1 solid piece, you could try placing a folded piece of paper under the base of it to straighten it out. Other than that, there is not much you can do. In the end, you might end up with a straight taper candle but an obviously crooked holder.