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 $234.87   13.192%   $265.8541   $265.85   $265.85   $265.90  
  Purchase #2 $29.84   12.786%   $33.6553   $33.66   $33.65   $33.70  
  Purchase #3 $45.43   5.520%   $47.9377   $47.94   $47.95   $47.90  
  Purchase #4 $250.72   10.102%   $276.0477   $276.05   $276.05   $276.00  
  Purchase #5 $91.52   6.507%   $97.4752   $97.48   $97.50   $97.50  
  Purchase #6 $197.89   10.348%   $218.3677   $218.37   $218.35   $218.40  
  Purchase #7 $69.50   9.523%   $76.1185   $76.12   $76.10   $76.10  
  7 purchase totals = $919.77       $1,015.4562   $1,015.47   $1,015.45   $1,015.50