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 $142.89   12.800%   $161.1799   $161.18   $161.20   $161.20  
  Purchase #2 $64.94   3.527%   $67.2304   $67.23   $67.25   $67.20  
  Purchase #3 $160.88   5.688%   $170.0309   $170.03   $170.05   $170.00  
  Purchase #4 $85.92   4.635%   $89.9024   $89.90   $89.90   $89.90  
  Purchase #5 $45.55   7.182%   $48.8214   $48.82   $48.80   $48.80  
  Purchase #6 $129.31   9.201%   $141.2078   $141.21   $141.20   $141.20  
  Purchase #7 $106.40   8.189%   $115.1131   $115.11   $115.10   $115.10  
  7 purchase totals = $735.89       $793.4859   $793.48   $793.50   $793.40