The stinger is intended to be used to transmit force only along the axis of the stinger. It is very difficult to identify the exact level of inaccuracy that can be absorbed by a stinger. The thicker the stinger, the more misalignment it can physically support, but the more transverse force will be input into the system resulting in measurement error. The thinner the stinger, the more likely that any misalignment will bend the stinger, damaging it. However, since stingers are (relatively) inexpensive and intended to essentially be a mechanical fuse in your test setup, it is better to have an error in this direction. If you continue to bend stingers, damaging them beyond future use, it would follow that you have too much misalignment. At some point the quality of the measured data is suspect as well; however, this is dependent not just on the stinger but also on the structure under test and the frequency range to be considered.
(In a future updates of these FAQ, additional information with quantified example data will be provided for further understanding of this topic.)