It depends on the breed of your dog actually. Sheltland Sheepdogs (Shelties), Aussies, Rough Collies and other herding dogs can have a genetic defect that causes their livers to improperly metabolize Ivermectin - found in Heartgard and other heartworm medications. For those breeds, a genetic blood test can be performed to see if the dog is positive for this defect - the MDR1 gene. If he/she is, then any drug with Ivermectin in it can cause seizures, neurological disorders and even death.
These dogs used to take the drug Interceptor, however the manufacturer quit making the product, leaving the herding group community out in the cold. Most of my Sheltie friends have tested their dogs for the MDR1 gene, as have I. Three of my shelties are free of the gene and are on Heartgard. The fourth is still a pup, and he hasn't been tested. He's on Sentinel, which uses the Interceptor drug plus flea medications.
I, personally, don't like giving my dogs a lot of chemicals, so the flea/tick medication is undesirable to me. I treat for fleas when I find them, which is only in September of each year. As my pup is too young to sit still for an easy blood draw and as it's nearing September, I'm putting him on the Sentinel for two months until he grows up enough to test for the MDR1 gene.
If your dog is a herding dog or a mix thereof, I suggest getting him tested for the MDR1 gene. Otherwise, I suggest going with a stand alone product that only treats heartworms especially if your dog is in a heavily populated metro area where ticks will be rare. Then you can treat for fleas as needed. If you are in a rural area, then tick preventative is vital, and a combo product may be required.