Yep syns are flexible like that, but I always make sure I "save them up" before using extra at the weekend rather than making up for it later, that way I know I wont cheat and let myself forget about them.
Some consultants say that this is fine and others dont. My Consultant doesnt think its a good idea, something to do with a metabolism. I think if you have your "naughty" day a few days before WI then you should be fine.
I eat what I like after WI. I usually have a takeaway and a little pud and then back to normal the next day. Try it and see if it affects your losses. I think you will be fine!