Hi ,Arun,
I did not understand - what is Your final product.

If its some UPS supervisor - higher level of safety (of device functioning) is desired. I do not claim that opto is best decision - You will take decision.
For such device i think transformer is best , but its really costly and big.

If device is dimmer - first of all - how many wires will go to them - 2 or 3 ? If it's not permanently powered by mains (2 wires only instead 3 wires) - 1..2,5 mA is limit for consumption - and 89s52 is far away from this point.If You have 3 wires 89s52 can be used but powering is still problem - 10...20mA results in big head dissipation, unless You use SMPS or trafo.
But if You use SMPS or trafo, then ZCD via resistor is non-elegant.
And price of optocoupler is not so important , even in 100k series - quality of product is more important.

My friend produces 2-wire dimmer with PIC12f509 , RC powering, triac and infrared receiver, mounted instead wall switch(i'm involved in firmware here). For 8k devices sold, i didn't know any problems with R,C,micro or IR , caused by high voltage spices . Some devices come back with dead triac - but what exactly is reason , hi dont know. (End of life in incandescent lamps is really high power event :-) .

As about 25V with 2 optocouplers..... If You use power enough micro with 2..3 timers - You can measure period , length of ON and OFF states and calculate where exactly is zero crossing. And You can do this permanently.

regards