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 $5.79   8.453%   $6.2794   $6.28   $6.30   $6.30  
  Purchase #2 $21.60   5.805%   $22.8539   $22.85   $22.85   $22.90  
  Purchase #3 $66.85   12.849%   $75.4396   $75.44   $75.45   $75.40  
  Purchase #4 $159.85   11.105%   $177.6013   $177.60   $177.60   $177.60  
  Purchase #5 $69.34   11.161%   $77.079   $77.08   $77.10   $77.10  
  Purchase #6 $170.33   6.296%   $181.054   $181.05   $181.05   $181.10  
  Purchase #7 $30.60   7.316%   $32.8387   $32.84   $32.85   $32.80  
  7 purchase totals = $524.36       $573.1459   $573.14   $573.20   $573.20