Ahhh yeah know what you mean. I'd do it intermittently that way she doesn't get a routine built up, it's quite good if you finish at different times then that way she doesn't get used to a time, but I wouldn't make it a nightly thing as again she'll come to expect it. I do the same with ours they never know when they're going to get a treat out, but I have to be careful as Dexter started getting into the habit of zooming out of the front door whenever it was opened so I always take him out the back door and make a lot of loud noise when I come in the front that way he knows the front door is not a good place to go out, but the back is where he can go when his harness comes out. Phew sorry bit of a marathon text.
Keep it very now and again that way she gets the best of both worlds without building up her expectations.