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 $123.32   12.870%   $139.1913   $139.19   $139.20   $139.20  
  Purchase #2 $139.85   3.337%   $144.5168   $144.52   $144.50   $144.50  
  Purchase #3 $36.39   11.430%   $40.5494   $40.55   $40.55   $40.50  
  Purchase #4 $117.43   3.438%   $121.4672   $121.47   $121.45   $121.50  
  Purchase #5 $48.41   10.476%   $53.4814   $53.48   $53.50   $53.50  
  Purchase #6 $121.31   10.562%   $134.1228   $134.12   $134.10   $134.10  
  Purchase #7 $8.34   8.834%   $9.0768   $9.08   $9.10   $9.10  
  7 purchase totals = $595.05       $642.4057   $642.41   $642.40   $642.40