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 $155.79   8.253%   $168.6473   $168.65   $168.65   $168.60  
  Purchase #2 $207.66   13.724%   $236.1593   $236.16   $236.15   $236.20  
  Purchase #3 $64.86   13.779%   $73.7971   $73.80   $73.80   $73.80  
  Purchase #4 $83.65   3.235%   $86.3561   $86.36   $86.35   $86.40  
  Purchase #5 $39.36   13.378%   $44.6256   $44.63   $44.65   $44.60  
  Purchase #6 $31.89   5.496%   $33.6427   $33.64   $33.65   $33.60  
  Purchase #7 $186.92   5.705%   $197.5838   $197.58   $197.60   $197.60  
  7 purchase totals = $770.13       $840.8119   $840.82   $840.85   $840.80