Windows that are out of square are a fact of life. Whether your home is old or new, mass produced or custom built, chances are you will have at least one window that is out of square. We see this continually, and it is always a challenge for my installers to make the window treatments look perfect.
There are a number of window treatments that are not recommended for badly irregular windows, if alternatives are available.
Plantation shutters are rigid structures, and as such, have very little room for movement in an out of square window. At least if timber is used, the fitter will possibly be able to plane certain parts to fit, otherwise lots of caulk will need to be used. Polymers and aluminium cannot be altered on site to accommodate window anomalies.
Roller blinds are also repeat offenders. They are designed to roll down in a straight line and if the window is crooked, so the blind will also appear crooked. A competent installer can add some packing to the blind, but only to gain a maximum of 10mm; any more than this and the blind will both sit perfectly.
Probably the best solution is a soft treatment and something that sits on the face of the window to actually hide the irregularities.
Roman blinds are OK, but curtains are definitely the best solution. Curtains can be extended higher than the window and extend further on either side of the frame, thus camouflaging crookedness. However, one also has to be mindful of the horizontals, so that the curtain looks level with the floor and the ceiling.
Rather than using rods, perhaps considering a pelmet which will conceal the tracking and allow the installer to adjust the hook position on the curtains, would be the ideal solution.
Remember, if your windows are out of square, use some common sense;rigid treatments cannot generally be adjusted to make the window perfect. Sometimes in life we just have to accept what is.