Although I am a very low stimulant user, you have to put the amount which is in a supp in perspective.
In our Pre Workout NT we only have 100mg of caffeine in there per serve which is what would be considered an average "brewed" coffee.
Many people have 5 or more cups of coffee a day, which means there total stimulant intake would be many times more than having a supp like our pre workout NT twice a day.
Although DMAA is also added, the side effects from this don't appear to be as pronounced as those from caffeine.
Basically, a supplement like our pre workout NT is fine to be used every training day (very few people train more than 5 days a week) but if doing so it is bets to reduce your intake of coffee and other stims as much as possible.
When using it, the best way to tell if you are being desensitised is dosage. If you find you are using larger doses all the time, it doesn't hurt to have a week off noa and then.
Timing is also very important, if you train in the morning or after work this would be fine, however if you were a late night trainer, then I'd advise using a pre workout product with any stims as it could lead to insomnia.