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 $19.40   8.290%   $21.0083   $21.01   $21.00   $21.00  
  Purchase #2 $11.86   3.613%   $12.2885   $12.29   $12.30   $12.30  
  Purchase #3 $13.38   11.768%   $14.9546   $14.95   $14.95   $15.00  
  Purchase #4 $169.52   3.256%   $175.0396   $175.04   $175.05   $175.00  
  Purchase #5 $153.36   3.109%   $158.128   $158.13   $158.15   $158.10  
  Purchase #6 $18.59   3.451%   $19.2315   $19.23   $19.25   $19.20  
  Purchase #7 $225.84   4.708%   $236.4725   $236.47   $236.45   $236.50  
  7 purchase totals = $611.95       $637.1230   $637.12   $637.15   $637.10