Cruise RPM setting

Shortly after getting my private license I went to a different FBO and
got checked out in their aircrafts - C172 and Piper Cherokee.

I talked with the CFI and asked what RPM settings he would recommend.
I'm renting dry, so one way to reduce cost is to slow down and thus
save fuel. At the present time I'm just building hours towards my
commercial ticket. Rarely am I in a hurry to get anywhere. He
mentioned that if I needed to get somewhere quickly, to keep it around
2400. If I'm just building time then to reduce to 2300. It will save
fuel as well as reduce the sound level.

The CFI no longer flies out of that airport, so I don't have contact
with him. I'm interested in others opinions related to rpm settings.
Is there any harm to the engine with reducing the RPM even lower, say
2200 or even 2100 RPM. I will confess to bringing it back to as low
as 1800 for a short X-C and found it to be good slow flight practice.,
which gave my one leg a work out in order to keep the bubble in the
middle. I wouldn't want to make many flights at 1800 though.

In the 172 I have reduced fuel from over 7.5 gph at 2400rpm to about
5.5 gph at 2200-2300.

Thanks,

John