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 $107.65   13.282%   $121.9481   $121.95   $121.95   $121.90  
  Purchase #2 $54.26   10.636%   $60.0311   $60.03   $60.05   $60.00  
  Purchase #3 $37.44   12.198%   $42.0069   $42.01   $42.00   $42.00  
  Purchase #4 $228.32   8.776%   $248.3574   $248.36   $248.35   $248.40  
  Purchase #5 $100.43   3.862%   $104.3086   $104.31   $104.30   $104.30  
  Purchase #6 $208.92   8.128%   $225.901   $225.90   $225.90   $225.90  
  Purchase #7 $94.37   10.699%   $104.4666   $104.47   $104.45   $104.50  
  7 purchase totals = $831.39       $907.0197   $907.03   $907.00   $907.00