Is there any long term dangers of having a constant regular sinus rythym, yet slightly tachycardic heart rate?

Probably not: There is a tachycardia-induced cardiomyopathy which is occasionally seen in people with a chronic tachycardia. The exact rate necessary to produce the problem is not clear but it would be less likely to occur with a "mild" tachycardia. When there is tachycardia, though, we always look for the cause. If your thyroid gland is overactive for instance that needs to be treated.

Review causes: The long term dangers arise from the cause rather than sinus tachycardia itself. Some causes include deconditioning due to lack of physical activity, being overweight, having anemia, nicotine/ excessive alcohol/ excessive caffeine, hyperactive thyroid, chronic lung disease or chronic pain. Please see if any these apply to you and talk to your doctor about it.