Well... your heart rate shouldn't be randomly increasing for no reason. For what reason is it happening? Did you take something or do something that may be causing this? And if so, was it something serious? Because if that's teh case, then you probably shouldn't wait... Either way... I would say when you're hitting towards the 90-100 range for no particular reason.. I would kind of wonder why... Just remember, it's better safe than sorry. If you think somethings wrong, the sooner you act, the sooner they can help if it's something serious.