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 $227.87   12.479%   $256.3059   $256.31   $256.30   $256.30  
  Purchase #2 $146.68   8.376%   $158.9659   $158.97   $158.95   $159.00  
  Purchase #3 $129.43   13.217%   $146.5368   $146.54   $146.55   $146.50  
  Purchase #4 $206.30   11.855%   $230.7569   $230.76   $230.75   $230.80  
  Purchase #5 $81.60   6.350%   $86.7816   $86.78   $86.80   $86.80  
  Purchase #6 $29.86   6.206%   $31.7131   $31.71   $31.70   $31.70  
  Purchase #7 $12.46   13.695%   $14.1664   $14.17   $14.15   $14.20  
  7 purchase totals = $834.20       $925.2266   $925.24   $925.20   $925.30