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 $106.88   13.788%   $121.6166   $121.62   $121.60   $121.60  
  Purchase #2 $42.37   10.440%   $46.7934   $46.79   $46.80   $46.80  
  Purchase #3 $66.83   6.478%   $71.1592   $71.16   $71.15   $71.20  
  Purchase #4 $239.47   3.261%   $247.2791   $247.28   $247.30   $247.30  
  Purchase #5 $110.71   12.461%   $124.5056   $124.51   $124.50   $124.50  
  Purchase #6 $134.73   7.855%   $145.313   $145.31   $145.30   $145.30  
  Purchase #7 $55.96   10.590%   $61.8862   $61.89   $61.90   $61.90  
  7 purchase totals = $756.95       $818.5531   $818.56   $818.55   $818.60