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 $57.52   11.828%   $64.3235   $64.32   $64.30   $64.30  
  Purchase #2 $11.97   3.314%   $12.3667   $12.37   $12.35   $12.40  
  Purchase #3 $109.28   11.493%   $121.8396   $121.84   $121.85   $121.80  
  Purchase #4 $58.42   12.827%   $65.9135   $65.91   $65.90   $65.90  
  Purchase #5 $215.32   4.780%   $225.6123   $225.61   $225.60   $225.60  
  Purchase #6 $112.30   5.379%   $118.3406   $118.34   $118.35   $118.30  
  Purchase #7 $118.73   11.144%   $131.9613   $131.96   $131.95   $132.00  
  7 purchase totals = $683.54       $740.3575   $740.35   $740.30   $740.30