Dectomax contains doromectin which is a wormer not dissimilar to ivermectin - the chemical also treats for scab.
So: If you have scab, it is a good option, specially as it can clear it up in one dose. If you don't have scab, Dectomax is expensive and you might as well use ivermectin. I wouldn't bother using both, it seems mildly pointless.
I don't know what flukicide is best to use in your area - triclabenzadole is reccomended here, but in some areas resistant fluke exist. I certainly wouldnt be using a combined flukicide/wormer and dectomax. You might as well use a flukicide and a wormer.
Generally speaking, treating things 'just in case' is how resistance to drugs is becoming more widespread.
Were I you, I would: Fluke them now with whatever flukicide works in your are, have a worm count done and worm accordingly when the results are back.
Unless of course, your sheep have scab, in which case, Dectomax it is, but certainly not Dectomax and a combined wormer/flukicide.
Edited to add: 'by eye' observation of eggs in faeces gives you no idea of the relative burden, because it is in no way quantitative - all sheep will have some kind of worms present, most of the time. It also negates to inform you which of the microscopic species are present.