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 $92.85   4.607%   $97.1276   $97.13   $97.15   $97.10  
  Purchase #2 $140.78   13.137%   $159.2743   $159.27   $159.25   $159.30  
  Purchase #3 $46.37   5.389%   $48.8689   $48.87   $48.85   $48.90  
  Purchase #4 $205.87   10.409%   $227.299   $227.30   $227.30   $227.30  
  Purchase #5 $61.84   8.823%   $67.2961   $67.30   $67.30   $67.30  
  Purchase #6 $124.51   12.840%   $140.4971   $140.50   $140.50   $140.50  
  Purchase #7 $230.82   11.641%   $257.6898   $257.69   $257.70   $257.70  
  7 purchase totals = $903.04       $998.0528   $998.06   $998.05   $998.10