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	$37.38	13.255%	$42.3347	$42.33	$42.35	$42.30
Purchase #2	$222.73	12.463%	$250.4888	$250.49	$250.50	$250.50
Purchase #3	$223.76	8.431%	$242.6252	$242.63	$242.65	$242.60
Purchase #4	$188.99	8.184%	$204.4569	$204.46	$204.45	$204.50
Purchase #5	$49.44	9.498%	$54.1358	$54.14	$54.15	$54.10
Purchase #6	$35.74	6.147%	$37.9369	$37.94	$37.95	$37.90
Purchase #7	$86.87	12.659%	$97.8669	$97.87	$97.85	$97.90
7 purchase totals =	$844.91		$929.8452	$929.86	$929.90	$929.80