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 $14.83   10.138%   $16.3335   $16.33   $16.35   $16.30  
  Purchase #2 $171.63   10.625%   $189.8657   $189.87   $189.85   $189.90  
  Purchase #3 $10.27   5.177%   $10.8017   $10.80   $10.80   $10.80  
  Purchase #4 $160.74   9.399%   $175.848   $175.85   $175.85   $175.80  
  Purchase #5 $236.69   6.259%   $251.5044   $251.50   $251.50   $251.50  
  Purchase #6 $83.69   8.348%   $90.6764   $90.68   $90.70   $90.70  
  Purchase #7 $236.95   12.612%   $266.8341   $266.83   $266.85   $266.80  
  7 purchase totals = $914.80       $1,001.8638   $1,001.86   $1,001.90   $1,001.80