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 $25.90   11.642%   $28.9153   $28.92   $28.90   $28.90  
  Purchase #2 $91.50   3.368%   $94.5817   $94.58   $94.60   $94.60  
  Purchase #3 $112.82   9.415%   $123.442   $123.44   $123.45   $123.40  
  Purchase #4 $14.59   7.439%   $15.6754   $15.68   $15.70   $15.70  
  Purchase #5 $113.34   7.425%   $121.7555   $121.76   $121.75   $121.80  
  Purchase #6 $206.50   4.197%   $215.1668   $215.17   $215.15   $215.20  
  Purchase #7 $38.69   11.294%   $43.0596   $43.06   $43.05   $43.10  
  7 purchase totals = $603.34       $642.5963   $642.61   $642.60   $642.70