Should only single hair grafts be used in the front of the hairline? 5 months post-op.

The hairline should be done in 100% single hairs in the proper angle and direction in order to get natural result. Anything more than single hair, esp if have coarser hair, and you will get an unnatural outcome that looks pluggy.
The triples and double can be removed by FUE and reused behind the hairline. Unfortunately many clinics that don't use microscope during cutting and magnification during placing of grafts will end up using doubles and triples instead of singles.

Yes, the hairline should be made up of at least 90% single grafts. A hairline filled with double's and triples will look unnatural and pluggy. The Hair Transplant Mentor Joe Tillman did a great video on this. I have attached it below. It's a must watch. 

Grafts in the front of the hairline should be single-hair.  At 5 1/2 months you may be seeing grafts close to one another as opposed to multiple hair grafts.  A visit to your surgeon for an examination and to discuss your concerns would be warranted.

In order to make a hair transplant look natural in the front, 1-2 hair grafts can be used and the design should be slightly uneven.  This is because no one has a straight line when you look at the hairline.  A follow-up with your surgeon to discuss your concerns is warranted as they will know what hairs were placed in the design of the hairline. 

The frontal hairline should be created with only single grafts to produce a no-hairline hairline which means that the forehead transitions to a full head of hair without seeing a hairline