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 $246.89   13.512%   $280.2498   $280.25   $280.25   $280.20  
  Purchase #2 $221.28   6.286%   $235.1897   $235.19   $235.20   $235.20  
  Purchase #3 $163.53   13.531%   $185.6572   $185.66   $185.65   $185.70  
  Purchase #4 $171.40   5.199%   $180.3111   $180.31   $180.30   $180.30  
  Purchase #5 $232.84   13.669%   $264.6669   $264.67   $264.65   $264.70  
  Purchase #6 $168.79   8.182%   $182.6004   $182.60   $182.60   $182.60  
  Purchase #7 $42.46   5.527%   $44.8068   $44.81   $44.80   $44.80  
  7 purchase totals = $1,247.19       $1,373.4819   $1,373.49   $1,373.45   $1,373.50