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 $227.59   10.284%   $250.9954   $251.00   $251.00   $251.00  
  Purchase #2 $189.93   7.158%   $203.5252   $203.53   $203.55   $203.50  
  Purchase #3 $135.26   5.225%   $142.3273   $142.33   $142.35   $142.30  
  Purchase #4 $238.26   7.724%   $256.6632   $256.66   $256.65   $256.70  
  Purchase #5 $138.76   5.412%   $146.2697   $146.27   $146.25   $146.30  
  Purchase #6 $200.35   11.238%   $222.8653   $222.87   $222.85   $222.90  
  Purchase #7 $103.39   13.790%   $117.6475   $117.65   $117.65   $117.60  
  7 purchase totals = $1,233.54       $1,340.2936   $1,340.31   $1,340.30   $1,340.30