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 $82.40   9.723%   $90.4118   $90.41   $90.40   $90.40  
  Purchase #2 $43.42   4.768%   $45.4903   $45.49   $45.50   $45.50  
  Purchase #3 $197.26   5.534%   $208.1764   $208.18   $208.20   $208.20  
  Purchase #4 $156.29   12.866%   $176.3983   $176.40   $176.40   $176.40  
  Purchase #5 $201.93   3.579%   $209.1571   $209.16   $209.15   $209.20  
  Purchase #6 $198.30   9.497%   $217.1326   $217.13   $217.15   $217.10  
  Purchase #7 $41.61   6.791%   $44.4357   $44.44   $44.45   $44.40  
  7 purchase totals = $921.21       $991.2022   $991.21   $991.25   $991.20