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 $168.97   12.257%   $189.6807   $189.68   $189.70   $189.70  
  Purchase #2 $205.63   10.121%   $226.4418   $226.44   $226.45   $226.40  
  Purchase #3 $56.46   5.473%   $59.5501   $59.55   $59.55   $59.60  
  Purchase #4 $217.97   8.786%   $237.1208   $237.12   $237.10   $237.10  
  Purchase #5 $160.72   8.433%   $174.2735   $174.27   $174.25   $174.30  
  Purchase #6 $152.63   7.284%   $163.7476   $163.75   $163.75   $163.70  
  Purchase #7 $206.77   12.563%   $232.7465   $232.75   $232.75   $232.70  
  7 purchase totals = $1,169.15       $1,283.5610   $1,283.56   $1,283.55   $1,283.50