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	$121.32	13.174%	$137.3027	$137.30	$137.30	$137.30
Purchase #2	$44.27	4.274%	$46.1621	$46.16	$46.15	$46.20
Purchase #3	$38.95	9.197%	$42.5322	$42.53	$42.55	$42.50
Purchase #4	$55.48	12.538%	$62.4361	$62.44	$62.45	$62.40
Purchase #5	$133.60	3.837%	$138.7262	$138.73	$138.75	$138.70
Purchase #6	$53.42	5.740%	$56.4863	$56.49	$56.50	$56.50
Purchase #7	$168.82	10.489%	$186.5275	$186.53	$186.55	$186.50
7 purchase totals =	$615.86		$670.1731	$670.18	$670.25	$670.10