Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$246.56	13.279%	$279.3007	$279.30	$279.30	$279.30
Purchase #2	$40.28	6.254%	$42.7991	$42.80	$42.80	$42.80
Purchase #3	$39.57	8.169%	$42.8025	$42.80	$42.80	$42.80
Purchase #4	$45.37	5.640%	$47.9289	$47.93	$47.95	$47.90
Purchase #5	$136.54	6.328%	$145.1803	$145.18	$145.20	$145.20
Purchase #6	$179.96	9.226%	$196.5631	$196.56	$196.55	$196.60
Purchase #7	$161.87	5.380%	$170.5786	$170.58	$170.60	$170.60
7 purchase totals =	$850.15		$925.1532	$925.15	$925.20	$925.20