If your speakers impedance doesn't swing too wildly and/or deeply below 4 ohms, then you may not need to worry unless you have a tube amp without 4 ohm taps.
But it all depends on how loudly you like to listen.
Now if you're talking about SET amp, that's a different story. SET tube amps have limited power & like smooth impedance curves above 8 ohms for the most part.
Read this article by Welbourne Labs (http://www.welbornelabs.com/recomendspeaks.htm). Be sure to scroll down & take particular notice of article "Some Insight into Proper Speaker Selection."
But it all depends on how loudly you like to listen.
Now if you're talking about SET amp, that's a different story. SET tube amps have limited power & like smooth impedance curves above 8 ohms for the most part.
Read this article by Welbourne Labs (http://www.welbornelabs.com/recomendspeaks.htm). Be sure to scroll down & take particular notice of article "Some Insight into Proper Speaker Selection."