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 $5.31   9.457%   $5.8122   $5.81   $5.80   $5.80  
  Purchase #2 $137.26   12.543%   $154.4765   $154.48   $154.50   $154.50  
  Purchase #3 $110.29   10.239%   $121.5826   $121.58   $121.60   $121.60  
  Purchase #4 $176.60   13.258%   $200.0136   $200.01   $200.00   $200.00  
  Purchase #5 $200.30   13.300%   $226.9399   $226.94   $226.95   $226.90  
  Purchase #6 $69.70   8.742%   $75.7932   $75.79   $75.80   $75.80  
  Purchase #7 $122.67   9.426%   $134.2329   $134.23   $134.25   $134.20  
  7 purchase totals = $822.13       $918.8509   $918.84   $918.90   $918.80