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 $128.40   11.270%   $142.8707   $142.87   $142.85   $142.90  
  Purchase #2 $86.68   10.277%   $95.5881   $95.59   $95.60   $95.60  
  Purchase #3 $163.63   13.371%   $185.509   $185.51   $185.50   $185.50  
  Purchase #4 $12.85   6.495%   $13.6846   $13.68   $13.70   $13.70  
  Purchase #5 $25.57   3.405%   $26.4407   $26.44   $26.45   $26.40  
  Purchase #6 $78.33   9.715%   $85.9398   $85.94   $85.95   $85.90  
  Purchase #7 $84.80   8.407%   $91.9291   $91.93   $91.95   $91.90  
  7 purchase totals = $580.26       $641.9620   $641.96   $642.00   $641.90