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 $245.46   3.105%   $253.0815   $253.08   $253.10   $253.10  
  Purchase #2 $218.79   13.111%   $247.4756   $247.48   $247.50   $247.50  
  Purchase #3 $7.45   11.769%   $8.3268   $8.33   $8.35   $8.30  
  Purchase #4 $42.56   4.376%   $44.4224   $44.42   $44.40   $44.40  
  Purchase #5 $13.76   3.653%   $14.2627   $14.26   $14.25   $14.30  
  Purchase #6 $218.33   9.569%   $239.222   $239.22   $239.20   $239.20  
  Purchase #7 $30.92   7.187%   $33.1422   $33.14   $33.15   $33.10  
  7 purchase totals = $777.27       $839.9332   $839.93   $839.95   $839.90