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	$36.93	13.760%	$42.0116	$42.01	$42.00	$42.00
Purchase #2	$194.93	11.513%	$217.3723	$217.37	$217.35	$217.40
Purchase #3	$88.31	3.120%	$91.0653	$91.07	$91.05	$91.10
Purchase #4	$201.80	5.634%	$213.1694	$213.17	$213.15	$213.20
Purchase #5	$115.75	10.634%	$128.0589	$128.06	$128.05	$128.10
Purchase #6	$39.38	9.811%	$43.2436	$43.24	$43.25	$43.20
Purchase #7	$172.33	13.862%	$196.2184	$196.22	$196.20	$196.20
7 purchase totals =	$849.43		$931.1395	$931.14	$931.05	$931.20