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 $27.31   13.473%   $30.9895   $30.99   $31.00   $31.00  
  Purchase #2 $80.67   9.803%   $88.5781   $88.58   $88.60   $88.60  
  Purchase #3 $91.88   12.587%   $103.4449   $103.44   $103.45   $103.40  
  Purchase #4 $27.82   3.625%   $28.8285   $28.83   $28.85   $28.80  
  Purchase #5 $172.87   3.608%   $179.1071   $179.11   $179.10   $179.10  
  Purchase #6 $71.51   3.203%   $73.8005   $73.80   $73.80   $73.80  
  Purchase #7 $139.63   4.655%   $146.1298   $146.13   $146.15   $146.10  
  7 purchase totals = $611.69       $650.8784   $650.88   $650.95   $650.80