Sure you can have a cheat meal every now and then. If you eat clean 80-90% of the time, a cheat meal or cheat day is good for the body.
That said, do not let a cheat turn into a binge. Also I would advise towards eating the most "healthy" junk food you can.
Instead of regular corn chips... stone-ground organic corn chips
The difference is a bit more fiber, and healthier oils. Sometimes you will find they have a bit less oil and salt too though that depends on brand. Also they are trans fat free.
Or like pizza? Make your own.
Get hormone and antibiotic free cheese/meats for the pizza, organic tomato sauce, and if you enjoy it whole grain dough. If you are into sausage/pepperoni then find some without nitrates/nitrites.
Trans fats, MSG, hormones and antibiotics found in animal products, and to a lesser degree various chemical additives will slow progress and threaten health.