Depends what treatment you get for your psoriasis. If steroid creams (or ointments) are used the steroid can be absorbed into the bloodstream an may affect your BG by a slight raise. Certainly not as bad effect as high dose steroid tablets, which I have had to take in the past for other conditions, which require me to almost double my basal insulin dose, and increase bolus doses.
I have used steroid creams because of occasional outbreaks of discoid eczema, but without any noticeable effect on my BG. None of the other treatments for psoriasis will have any effect on your BG, so don't worry.