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 $241.60   13.398%   $273.9696   $273.97   $273.95   $274.00  
  Purchase #2 $50.68   8.807%   $55.1434   $55.14   $55.15   $55.10  
  Purchase #3 $55.72   13.266%   $63.1118   $63.11   $63.10   $63.10  
  Purchase #4 $246.72   13.389%   $279.7533   $279.75   $279.75   $279.80  
  Purchase #5 $126.90   9.531%   $138.9948   $138.99   $139.00   $139.00  
  Purchase #6 $225.34   11.377%   $250.9769   $250.98   $251.00   $251.00  
  Purchase #7 $129.34   10.311%   $142.6762   $142.68   $142.70   $142.70  
  7 purchase totals = $1,076.30       $1,204.6260   $1,204.62   $1,204.65   $1,204.70