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	$18.59	9.608%	$20.3761	$20.38	$20.40	$20.40
Purchase #2	$36.76	8.176%	$39.7655	$39.77	$39.75	$39.80
Purchase #3	$138.31	8.656%	$150.2821	$150.28	$150.30	$150.30
Purchase #4	$26.42	4.808%	$27.6903	$27.69	$27.70	$27.70
Purchase #5	$7.30	13.576%	$8.291	$8.29	$8.30	$8.30
Purchase #6	$113.73	6.774%	$121.4341	$121.43	$121.45	$121.40
Purchase #7	$81.49	9.242%	$89.0213	$89.02	$89.00	$89.00
7 purchase totals =	$422.60		$456.8604	$456.86	$456.90	$456.90