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 $76.48   13.594%   $86.8767   $86.88   $86.90   $86.90  
  Purchase #2 $183.27   11.132%   $203.6716   $203.67   $203.65   $203.70  
  Purchase #3 $136.46   13.200%   $154.4727   $154.47   $154.45   $154.50  
  Purchase #4 $46.35   11.855%   $51.8448   $51.84   $51.85   $51.80  
  Purchase #5 $205.33   9.493%   $224.822   $224.82   $224.80   $224.80  
  Purchase #6 $231.28   6.693%   $246.7596   $246.76   $246.75   $246.80  
  Purchase #7 $27.63   13.200%   $31.2772   $31.28   $31.30   $31.30  
  7 purchase totals = $906.80       $999.7246   $999.72   $999.70   $999.80