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 $230.51   11.313%   $256.5876   $256.59   $256.60   $256.60  
  Purchase #2 $227.56   13.107%   $257.3863   $257.39   $257.40   $257.40  
  Purchase #3 $65.32   13.792%   $74.3289   $74.33   $74.35   $74.30  
  Purchase #4 $94.45   10.648%   $104.507   $104.51   $104.50   $104.50  
  Purchase #5 $228.67   9.343%   $250.0346   $250.03   $250.05   $250.00  
  Purchase #6 $56.73   6.422%   $60.3732   $60.37   $60.35   $60.40  
  Purchase #7 $244.84   8.481%   $265.6049   $265.60   $265.60   $265.60  
  7 purchase totals = $1,148.08       $1,268.8225   $1,268.82   $1,268.85   $1,268.80