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	$37.37	12.592%	$42.0756	$42.08	$42.10	$42.10
Purchase #2	$102.88	12.587%	$115.8295	$115.83	$115.85	$115.80
Purchase #3	$200.88	11.490%	$223.9611	$223.96	$223.95	$224.00
Purchase #4	$127.35	13.726%	$144.8301	$144.83	$144.85	$144.80
Purchase #5	$165.75	11.833%	$185.3632	$185.36	$185.35	$185.40
Purchase #6	$223.48	7.488%	$240.2142	$240.21	$240.20	$240.20
Purchase #7	$185.80	9.403%	$203.2708	$203.27	$203.25	$203.30
7 purchase totals =	$1,043.51		$1,155.5445	$1,155.54	$1,155.55	$1,155.60