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 $248.27   12.746%   $279.9145   $279.91   $279.90   $279.90  
  Purchase #2 $121.99   11.280%   $135.7505   $135.75   $135.75   $135.80  
  Purchase #3 $238.87   6.123%   $253.496   $253.50   $253.50   $253.50  
  Purchase #4 $51.75   4.156%   $53.9007   $53.90   $53.90   $53.90  
  Purchase #5 $165.88   7.262%   $177.9262   $177.93   $177.95   $177.90  
  Purchase #6 $73.75   3.639%   $76.4338   $76.43   $76.45   $76.40  
  Purchase #7 $69.59   8.289%   $75.3583   $75.36   $75.35   $75.40  
  7 purchase totals = $970.10       $1,052.7800   $1,052.78   $1,052.80   $1,052.80