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 $28.82   13.382%   $32.6767   $32.68   $32.70   $32.70  
  Purchase #2 $178.44   11.472%   $198.9106   $198.91   $198.90   $198.90  
  Purchase #3 $80.32   3.581%   $83.1963   $83.20   $83.20   $83.20  
  Purchase #4 $229.63   6.158%   $243.7706   $243.77   $243.75   $243.80  
  Purchase #5 $62.86   5.374%   $66.2381   $66.24   $66.25   $66.20  
  Purchase #6 $241.40   11.813%   $269.9166   $269.92   $269.90   $269.90  
  Purchase #7 $92.71   12.280%   $104.0948   $104.09   $104.10   $104.10  
  7 purchase totals = $914.18       $998.8037   $998.81   $998.80   $998.80