It also depends on the amp. If it is a chip-amp, you don't need to worry, just connect it. They have current limiter safety circuits.
The thing is that every speaker, even 8 ohm ones, dip to 2 ohm now and then in bass parts. This is caused by the speaker generating current while moving back to the middle position. An amp has to deal with these resistance fluctuations. But if the amp is a classic transistor amp, you better use the extra resistor. Although you throw away half the power, and if you put up the volume, you end up using the same current for the same volume in the end, and the current is what kills the amp.
And you can also go down in resistance. Just put a resistor in parallel for that.
Two speakers in series is only OK when they are perfect the same species. If they are different models, they react different on the amp, and the return current generated will mess up the other of the 2 speakers.