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 $120.54   7.272%   $129.3057   $129.31   $129.30   $129.30  
  Purchase #2 $82.25   12.282%   $92.3519   $92.35   $92.35   $92.40  
  Purchase #3 $176.43   3.568%   $182.725   $182.73   $182.75   $182.70  
  Purchase #4 $217.31   4.782%   $227.7018   $227.70   $227.70   $227.70  
  Purchase #5 $228.80   10.537%   $252.9087   $252.91   $252.90   $252.90  
  Purchase #6 $142.64   3.666%   $147.8692   $147.87   $147.85   $147.90  
  Purchase #7 $29.71   13.524%   $33.728   $33.73   $33.75   $33.70  
  7 purchase totals = $997.68       $1,066.5903   $1,066.60   $1,066.60   $1,066.60