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 $173.60   4.156%   $180.8148   $180.81   $180.80   $180.80  
  Purchase #2 $51.26   11.534%   $57.1723   $57.17   $57.15   $57.20  
  Purchase #3 $213.37   6.323%   $226.8614   $226.86   $226.85   $226.90  
  Purchase #4 $171.81   7.353%   $184.4432   $184.44   $184.45   $184.40  
  Purchase #5 $18.39   7.708%   $19.8075   $19.81   $19.80   $19.80  
  Purchase #6 $48.31   10.605%   $53.4333   $53.43   $53.45   $53.40  
  Purchase #7 $192.98   12.474%   $217.0523   $217.05   $217.05   $217.10  
  7 purchase totals = $869.72       $939.5848   $939.57   $939.55   $939.60