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 $204.95   13.150%   $231.9009   $231.90   $231.90   $231.90  
  Purchase #2 $26.70   11.223%   $29.6965   $29.70   $29.70   $29.70  
  Purchase #3 $10.52   10.322%   $11.6059   $11.61   $11.60   $11.60  
  Purchase #4 $201.70   10.256%   $222.3864   $222.39   $222.40   $222.40  
  Purchase #5 $164.34   9.413%   $179.8093   $179.81   $179.80   $179.80  
  Purchase #6 $48.98   3.790%   $50.8363   $50.84   $50.85   $50.80  
  Purchase #7 $105.58   8.657%   $114.7201   $114.72   $114.70   $114.70  
  7 purchase totals = $762.77       $840.9554   $840.97   $840.95   $840.90