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	$28.39	10.861%	$31.4734	$31.47	$31.45	$31.50
Purchase #2	$232.99	12.202%	$261.4194	$261.42	$261.40	$261.40
Purchase #3	$91.83	7.782%	$98.9762	$98.98	$99.00	$99.00
Purchase #4	$35.44	13.625%	$40.2687	$40.27	$40.25	$40.30
Purchase #5	$146.44	10.834%	$162.3053	$162.31	$162.30	$162.30
Purchase #6	$18.47	8.351%	$20.0124	$20.01	$20.00	$20.00
Purchase #7	$6.64	7.354%	$7.1283	$7.13	$7.15	$7.10
7 purchase totals =	$560.20		$621.5837	$621.59	$621.55	$621.60