You can wet clean curtains with an upholstery tool or special curtain tool. You need a board to put behind the fabric, a plastic plasterers hawk is ideal.
You need to test for shrinkage by putting 4 pins through the fabric making a square. You measure the sides of the square, wet the fabric in that area and dry off with a hairdryer. Then you measure the sides of the square again to see if there has been any shrinkage.
You can also mist clean them with Microsplit and an upholstery sponge. Again using the plasterers hawk behind them.
You must always thoroughly examine them for pre-existing damage as the fabrics can deteriorate due to sunlight and thermal currents passing through, especially if there is a gas cooker in the house.
Or you can take them down and take them to the dry cleaners who you will have negotiated a discount from, usually 20-30%. Take them back and re-hang them and charge the customer the full amount.
No hassle, apart from the taking down and putting up and if they shrink the dry cleaners are responsible.
Basically curtains are more bother than they are worth in my opinion.