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 $37.40   6.596%   $39.8669   $39.87   $39.85   $39.90  
  Purchase #2 $61.29   11.628%   $68.4168   $68.42   $68.40   $68.40  
  Purchase #3 $202.76   6.266%   $215.4649   $215.46   $215.45   $215.50  
  Purchase #4 $187.95   13.449%   $213.2274   $213.23   $213.25   $213.20  
  Purchase #5 $48.71   7.496%   $52.3613   $52.36   $52.35   $52.40  
  Purchase #6 $32.95   10.704%   $36.477   $36.48   $36.50   $36.50  
  Purchase #7 $7.70   10.254%   $8.4896   $8.49   $8.50   $8.50  
  7 purchase totals = $578.76       $634.3039   $634.31   $634.30   $634.40