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	$19.76	6.310%	$21.0069	$21.01	$21.00	$21.00
Purchase #2	$97.69	4.571%	$102.1554	$102.16	$102.15	$102.20
Purchase #3	$55.87	6.800%	$59.6692	$59.67	$59.65	$59.70
Purchase #4	$44.87	4.253%	$46.7783	$46.78	$46.80	$46.80
Purchase #5	$214.57	5.800%	$227.0151	$227.02	$227.00	$227.00
Purchase #6	$129.72	5.611%	$136.9986	$137.00	$137.00	$137.00
Purchase #7	$85.87	12.155%	$96.3075	$96.31	$96.30	$96.30
7 purchase totals =	$648.35		$689.9310	$689.95	$689.90	$690.00