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 $231.85   13.457%   $263.0501   $263.05   $263.05   $263.10  
  Purchase #2 $60.58   6.707%   $64.6431   $64.64   $64.65   $64.60  
  Purchase #3 $33.34   6.848%   $35.6231   $35.62   $35.60   $35.60  
  Purchase #4 $140.66   7.766%   $151.5837   $151.58   $151.60   $151.60  
  Purchase #5 $72.99   9.379%   $79.8357   $79.84   $79.85   $79.80  
  Purchase #6 $134.75   12.724%   $151.8956   $151.90   $151.90   $151.90  
  Purchase #7 $211.54   10.821%   $234.4307   $234.43   $234.45   $234.40  
  7 purchase totals = $885.71       $981.0620   $981.06   $981.10   $981.00