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 $30.29   3.256%   $31.2762   $31.28   $31.30   $31.30  
  Purchase #2 $162.86   4.398%   $170.0226   $170.02   $170.00   $170.00  
  Purchase #3 $151.69   7.473%   $163.0258   $163.03   $163.05   $163.00  
  Purchase #4 $39.36   7.511%   $42.3163   $42.32   $42.30   $42.30  
  Purchase #5 $28.77   9.154%   $31.4036   $31.40   $31.40   $31.40  
  Purchase #6 $76.56   10.564%   $84.6478   $84.65   $84.65   $84.60  
  Purchase #7 $50.56   12.642%   $56.9518   $56.95   $56.95   $57.00  
  7 purchase totals = $540.09       $579.6441   $579.65   $579.65   $579.60