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 $143.47   4.478%   $149.8946   $149.89   $149.90   $149.90  
  Purchase #2 $75.78   3.217%   $78.2178   $78.22   $78.20   $78.20  
  Purchase #3 $27.54   9.452%   $30.1431   $30.14   $30.15   $30.10  
  Purchase #4 $180.60   6.590%   $192.5015   $192.50   $192.50   $192.50  
  Purchase #5 $16.37   10.773%   $18.1335   $18.13   $18.15   $18.10  
  Purchase #6 $143.38   11.451%   $159.7984   $159.80   $159.80   $159.80  
  Purchase #7 $12.40   13.724%   $14.1018   $14.10   $14.10   $14.10  
  7 purchase totals = $599.54       $642.7907   $642.78   $642.80   $642.70