Have a look at your CPU RPM readout under full CPU load and idle. It is true that if you have ACPI enabled your RPMs will increase under load, but only by 200-300 in my experience, certainly not enough to affect temps.
Also, dont forget the idle state takes a few minutes from boot up to lower itself down, so for example you might boot up at 50C and start surfing the web and then see that it has dropped down to 36C.
It depends what you mean by "when I start working"?