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 $37.50   3.692%   $38.8845   $38.88   $38.90   $38.90  
  Purchase #2 $139.90   6.633%   $149.1796   $149.18   $149.20   $149.20  
  Purchase #3 $98.77   13.618%   $112.2205   $112.22   $112.20   $112.20  
  Purchase #4 $78.45   8.564%   $85.1685   $85.17   $85.15   $85.20  
  Purchase #5 $34.94   3.742%   $36.2475   $36.25   $36.25   $36.20  
  Purchase #6 $71.42   12.569%   $80.3968   $80.40   $80.40   $80.40  
  Purchase #7 $16.68   11.489%   $18.5964   $18.60   $18.60   $18.60  
  7 purchase totals = $477.66       $520.6938   $520.70   $520.70   $520.70