O_K_
75% power is the maximum recommended power in cruice, and is a fairly high power setting. It puts you well above Va, and very close to Vno.
I'm not so sure about some of what you say. Who has made the max cruise at 75% power recommendation?
The Lycoming engines in the C172 (either O320 or IO360) so far as I know according to Mr Lycomings book are good for Full throttle 2700 RPM all day. i.e. 100% power. I wouldn't want to pay the fuel bill mind you.
75% is a pretty normal power setting.
There's absolutely nothing wrong with cruising above Va. In fact most light aircraft I can think of cruise above Va. The wording if I recall correctly is no full and abrupt control inputs above Va. This still means normal control inputs are OK.
However as you allude to, the speed/power setting being used by the OP is most likely to do with saving money, i.e.fuel.
wood73
The 124 kts you mention is probably true airspeed (TAS) not indicated (IAS). Only at sea level on a standard day are TAS and IAS the same.
For a 75% power setting most non turbo charged light aircraft will achieve their best TAS at around 6000 to 8000 feet. On a standard day an IAS of 100 kts at 8000 feet will give a TAS of about 113 KTAS. For your 124 KTAS you would need to indicate 110 KIAS