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	$25.97	11.503%	$28.9573	$28.96	$28.95	$29.00
Purchase #2	$117.59	5.274%	$123.7917	$123.79	$123.80	$123.80
Purchase #3	$41.88	4.820%	$43.8986	$43.90	$43.90	$43.90
Purchase #4	$24.37	6.435%	$25.9382	$25.94	$25.95	$25.90
Purchase #5	$41.57	9.712%	$45.6073	$45.61	$45.60	$45.60
Purchase #6	$26.65	4.623%	$27.882	$27.88	$27.90	$27.90
Purchase #7	$113.26	9.685%	$124.2292	$124.23	$124.25	$124.20
7 purchase totals =	$391.29		$420.3043	$420.31	$420.35	$420.30