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 $89.27   12.809%   $100.7046   $100.70   $100.70   $100.70  
  Purchase #2 $224.88   12.345%   $252.6414   $252.64   $252.65   $252.60  
  Purchase #3 $37.26   5.685%   $39.3782   $39.38   $39.40   $39.40  
  Purchase #4 $40.32   11.616%   $45.0036   $45.00   $45.00   $45.00  
  Purchase #5 $68.53   5.698%   $72.4348   $72.43   $72.45   $72.40  
  Purchase #6 $63.93   3.494%   $66.1637   $66.16   $66.15   $66.20  
  Purchase #7 $62.88   10.407%   $69.4239   $69.42   $69.40   $69.40  
  7 purchase totals = $587.07       $645.7502   $645.73   $645.75   $645.70