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 $235.59   6.111%   $249.9869   $249.99   $250.00   $250.00  
  Purchase #2 $96.58   6.305%   $102.6694   $102.67   $102.65   $102.70  
  Purchase #3 $65.91   8.502%   $71.5137   $71.51   $71.50   $71.50  
  Purchase #4 $83.46   4.857%   $87.5137   $87.51   $87.50   $87.50  
  Purchase #5 $235.82   3.335%   $243.6846   $243.68   $243.70   $243.70  
  Purchase #6 $211.77   13.508%   $240.3759   $240.38   $240.40   $240.40  
  Purchase #7 $26.71   4.655%   $27.9534   $27.95   $27.95   $28.00  
  7 purchase totals = $955.84       $1,023.6976   $1,023.69   $1,023.70   $1,023.80