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 $27.42   12.604%   $30.876   $30.88   $30.90   $30.90  
  Purchase #2 $200.91   13.627%   $228.288   $228.29   $228.30   $228.30  
  Purchase #3 $74.50   7.599%   $80.1613   $80.16   $80.15   $80.20  
  Purchase #4 $72.54   11.263%   $80.7102   $80.71   $80.70   $80.70  
  Purchase #5 $183.71   13.224%   $208.0038   $208.00   $208.00   $208.00  
  Purchase #6 $85.56   6.735%   $91.3225   $91.32   $91.30   $91.30  
  Purchase #7 $198.86   9.186%   $217.1273   $217.13   $217.15   $217.10  
  7 purchase totals = $843.50       $936.4891   $936.49   $936.50   $936.50