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 $36.26   11.218%   $40.3276   $40.33   $40.35   $40.30  
  Purchase #2 $106.79   10.412%   $117.909   $117.91   $117.90   $117.90  
  Purchase #3 $117.48   4.230%   $122.4494   $122.45   $122.45   $122.40  
  Purchase #4 $107.73   13.737%   $122.5289   $122.53   $122.55   $122.50  
  Purchase #5 $22.77   12.576%   $25.6336   $25.63   $25.65   $25.60  
  Purchase #6 $38.82   3.414%   $40.1453   $40.15   $40.15   $40.10  
  Purchase #7 $166.48   4.216%   $173.4988   $173.50   $173.50   $173.50  
  7 purchase totals = $596.33       $642.4926   $642.50   $642.55   $642.30