I just did a heater core swap and I think I left a ground strap off under the dash near the heater core...... But I can't remember if there really was a ground strap or I am just thinking there was?? If there was one and I left it off is it a big deal?

Do I have to take the dash apart again..........???? ha ha

 

I was able to find through the search that their was a ground wire from the firewall and heater core. Took less than 2 minutes to loosen up the dash enough to attach the ground strap......

 

A well grounded heater core and radiator is important to prevent stray power from causing electrolysis which turns coolant acidic and eats the cores.
You did the right thing to find the ground and reconnect it!