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 $250.58   5.832%   $265.1938   $265.19   $265.20   $265.20  
  Purchase #2 $22.31   8.387%   $24.1811   $24.18   $24.20   $24.20  
  Purchase #3 $60.32   8.729%   $65.5853   $65.59   $65.60   $65.60  
  Purchase #4 $221.56   9.838%   $243.3571   $243.36   $243.35   $243.40  
  Purchase #5 $21.91   6.577%   $23.351   $23.35   $23.35   $23.40  
  Purchase #6 $18.52   12.239%   $20.7867   $20.79   $20.80   $20.80  
  Purchase #7 $19.60   4.327%   $20.4481   $20.45   $20.45   $20.40  
  7 purchase totals = $614.80       $662.9031   $662.91   $662.95   $663.00