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 $34.61   13.805%   $39.3879   $39.39   $39.40   $39.40  
  Purchase #2 $154.64   4.137%   $161.0375   $161.04   $161.05   $161.00  
  Purchase #3 $59.56   10.176%   $65.6208   $65.62   $65.60   $65.60  
  Purchase #4 $211.39   3.295%   $218.3553   $218.36   $218.35   $218.40  
  Purchase #5 $227.59   13.358%   $257.9915   $257.99   $258.00   $258.00  
  Purchase #6 $26.63   9.714%   $29.2168   $29.22   $29.20   $29.20  
  Purchase #7 $77.78   10.872%   $86.2362   $86.24   $86.25   $86.20  
  7 purchase totals = $792.20       $857.8460   $857.86   $857.85   $857.80