I'm going to buy Yaesu FT-891 as my first transceiver.
Aleksander - Good choice! I also have the same rig.
I don't rely on the internal SWR meter of the rig - the rig meter is really rather crude, compared to the meter on a good Antenna Tuner. My antenna is a simple dipole fed with ladder line. I use a Tuner and match to all bands.
Now for you, you are correct when you say that you will also be using an untuned antenna and with high SWR much of the energy will be returned to your rig.
At five watts, however, you will not damage the rig. But you will lose much of that power to heat energy at high SWR. Not the best situation.
Since you are using an untuned dipole (fed by ladder line hopefully) you should be using an Antenna Tuner. A good AT will have a meter on it. I would rely on the meter on the Tuner.
You want to operate on FM? So, that would be on six meters band? Because I don't know of any FM operations on any lower band, perhaps some on 10 meters. Is this decision because of your license level?
Anyway, you obviously COULD operate with using just the rig's SWR meter. But why? Should be a meter on the Tuner.
When tuning, I take into account the fact that with a high SWR, the rig (most modern rigs do this) will automatically reduce power to protect the rig. That is, if the SWR is high and I have the rig set to 100 watts, the rig will automatically reduce power output to very low. I think around 10 watts.
So, when I tune to a new band, or even a new part of a current band, I will set my power way down (10 watts) to start tuning. Only when I have a match at 10 watts, do I up the power and retune. BTW I find that I need to retune when I raise the power. Don't know why this is the case.
My simple advice is that if you are using an untuned antenna, you should use an Antenna Tuner to match the antenna to the rig. The rig expects 50 ohms resistive, and the tuner converts whatever the antenna is presenting to what the rig expects. That is, if you have expertly used your Antenna Tuner to properly tune the antenna. And that is a whole nother question and reply. If you ask that, I will reply to that.
73, and happy hamming!