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	$24.37	12.328%	$27.3743	$27.37	$27.35	$27.40
Purchase #2	$36.92	6.297%	$39.2449	$39.24	$39.25	$39.20
Purchase #3	$233.71	13.191%	$264.5387	$264.54	$264.55	$264.50
Purchase #4	$55.50	5.296%	$58.4393	$58.44	$58.45	$58.40
Purchase #5	$240.67	6.535%	$256.3978	$256.40	$256.40	$256.40
Purchase #6	$38.80	3.454%	$40.1402	$40.14	$40.15	$40.10
Purchase #7	$156.31	3.715%	$162.1169	$162.12	$162.10	$162.10
7 purchase totals =	$786.28		$848.2521	$848.25	$848.25	$848.10