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 $24.25   8.577%   $26.3299   $26.33   $26.35   $26.30  
  Purchase #2 $131.34   3.294%   $135.6663   $135.67   $135.65   $135.70  
  Purchase #3 $234.65   8.503%   $254.6023   $254.60   $254.60   $254.60  
  Purchase #4 $68.94   13.129%   $77.9911   $77.99   $78.00   $78.00  
  Purchase #5 $78.27   10.155%   $86.2183   $86.22   $86.20   $86.20  
  Purchase #6 $250.26   10.611%   $276.8151   $276.82   $276.80   $276.80  
  Purchase #7 $116.52   6.167%   $123.7058   $123.71   $123.70   $123.70  
  7 purchase totals = $904.23       $981.3288   $981.34   $981.30   $981.30