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 $242.56   11.871%   $271.3543   $271.35   $271.35   $271.40  
  Purchase #2 $195.87   3.251%   $202.2377   $202.24   $202.25   $202.20  
  Purchase #3 $18.52   11.166%   $20.5879   $20.59   $20.60   $20.60  
  Purchase #4 $144.37   13.268%   $163.525   $163.53   $163.55   $163.50  
  Purchase #5 $241.63   3.249%   $249.4806   $249.48   $249.50   $249.50  
  Purchase #6 $33.77   3.514%   $34.9567   $34.96   $34.95   $35.00  
  Purchase #7 $63.70   3.204%   $65.7409   $65.74   $65.75   $65.70  
  7 purchase totals = $940.42       $1,007.8831   $1,007.89   $1,007.95   $1,007.90