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 $228.30   9.299%   $249.5296   $249.53   $249.55   $249.50  
  Purchase #2 $217.69   9.641%   $238.6775   $238.68   $238.70   $238.70  
  Purchase #3 $28.26   5.164%   $29.7193   $29.72   $29.70   $29.70  
  Purchase #4 $148.43   3.732%   $153.9694   $153.97   $153.95   $154.00  
  Purchase #5 $23.54   8.404%   $25.5183   $25.52   $25.50   $25.50  
  Purchase #6 $232.35   7.755%   $250.3687   $250.37   $250.35   $250.40  
  Purchase #7 $65.45   3.113%   $67.4875   $67.49   $67.50   $67.50  
  7 purchase totals = $944.02       $1,015.2703   $1,015.28   $1,015.25   $1,015.30