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 $226.96   8.834%   $247.0096   $247.01   $247.00   $247.00  
  Purchase #2 $138.89   5.659%   $146.7498   $146.75   $146.75   $146.70  
  Purchase #3 $38.43   3.616%   $39.8196   $39.82   $39.80   $39.80  
  Purchase #4 $32.40   12.771%   $36.5378   $36.54   $36.55   $36.50  
  Purchase #5 $30.33   13.381%   $34.3885   $34.39   $34.40   $34.40  
  Purchase #6 $246.83   6.542%   $262.9776   $262.98   $263.00   $263.00  
  Purchase #7 $25.82   13.854%   $29.3971   $29.40   $29.40   $29.40  
  7 purchase totals = $739.66       $796.8800   $796.89   $796.90   $796.80