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 $173.93   13.472%   $197.3618   $197.36   $197.35   $197.40  
  Purchase #2 $69.65   11.619%   $77.7426   $77.74   $77.75   $77.70  
  Purchase #3 $206.66   9.161%   $225.5921   $225.59   $225.60   $225.60  
  Purchase #4 $199.43   13.230%   $225.8146   $225.81   $225.80   $225.80  
  Purchase #5 $112.74   12.445%   $126.7705   $126.77   $126.75   $126.80  
  Purchase #6 $46.72   13.708%   $53.1244   $53.12   $53.10   $53.10  
  Purchase #7 $102.80   12.387%   $115.5338   $115.53   $115.55   $115.50  
  7 purchase totals = $911.93       $1,021.9398   $1,021.92   $1,021.90   $1,021.90