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 $6.36   12.390%   $7.148   $7.15   $7.15   $7.10  
  Purchase #2 $28.39   5.662%   $29.9974   $30.00   $30.00   $30.00  
  Purchase #3 $66.71   9.441%   $73.0081   $73.01   $73.00   $73.00  
  Purchase #4 $64.94   11.687%   $72.5295   $72.53   $72.55   $72.50  
  Purchase #5 $205.31   7.359%   $220.4188   $220.42   $220.40   $220.40  
  Purchase #6 $144.62   10.419%   $159.688   $159.69   $159.70   $159.70  
  Purchase #7 $211.25   7.183%   $226.4241   $226.42   $226.40   $226.40  
  7 purchase totals = $727.58       $789.2139   $789.22   $789.20   $789.10