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 $218.34   12.560%   $245.7635   $245.76   $245.75   $245.80  
  Purchase #2 $50.87   9.242%   $55.5714   $55.57   $55.55   $55.60  
  Purchase #3 $23.77   12.193%   $26.6683   $26.67   $26.65   $26.70  
  Purchase #4 $146.27   10.647%   $161.8434   $161.84   $161.85   $161.80  
  Purchase #5 $229.46   7.628%   $246.9632   $246.96   $246.95   $247.00  
  Purchase #6 $12.27   5.116%   $12.8977   $12.90   $12.90   $12.90  
  Purchase #7 $51.53   4.120%   $53.653   $53.65   $53.65   $53.70  
  7 purchase totals = $732.51       $803.3605   $803.35   $803.30   $803.50