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 $74.95   10.724%   $82.9876   $82.99   $83.00   $83.00  
  Purchase #2 $21.65   12.853%   $24.4327   $24.43   $24.45   $24.40  
  Purchase #3 $54.56   10.457%   $60.2653   $60.27   $60.25   $60.30  
  Purchase #4 $221.41   6.624%   $236.0762   $236.08   $236.10   $236.10  
  Purchase #5 $239.31   7.571%   $257.4282   $257.43   $257.45   $257.40  
  Purchase #6 $86.33   10.668%   $95.5397   $95.54   $95.55   $95.50  
  Purchase #7 $186.62   8.582%   $202.6357   $202.64   $202.65   $202.60  
  7 purchase totals = $884.83       $959.3654   $959.38   $959.45   $959.30