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 $34.39   13.527%   $39.0419   $39.04   $39.05   $39.00  
  Purchase #2 $209.98   13.502%   $238.3315   $238.33   $238.35   $238.30  
  Purchase #3 $24.90   8.684%   $27.0623   $27.06   $27.05   $27.10  
  Purchase #4 $133.90   5.239%   $140.915   $140.92   $140.90   $140.90  
  Purchase #5 $178.52   11.676%   $199.364   $199.36   $199.35   $199.40  
  Purchase #6 $206.99   8.866%   $225.3417   $225.34   $225.35   $225.30  
  Purchase #7 $55.64   11.337%   $61.9479   $61.95   $61.95   $61.90  
  7 purchase totals = $844.32       $932.0043   $932.00   $932.00   $931.90