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 $100.45   8.485%   $108.9732   $108.97   $108.95   $109.00  
  Purchase #2 $27.30   4.324%   $28.4805   $28.48   $28.50   $28.50  
  Purchase #3 $223.89   11.677%   $250.0336   $250.03   $250.05   $250.00  
  Purchase #4 $103.92   12.406%   $116.8123   $116.81   $116.80   $116.80  
  Purchase #5 $163.37   10.681%   $180.8195   $180.82   $180.80   $180.80  
  Purchase #6 $45.45   4.543%   $47.5148   $47.51   $47.50   $47.50  
  Purchase #7 $227.93   4.186%   $237.4711   $237.47   $237.45   $237.50  
  7 purchase totals = $892.31       $970.1050   $970.09   $970.05   $970.10