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 $115.62   10.709%   $128.0017   $128.00   $128.00   $128.00  
  Purchase #2 $25.65   9.636%   $28.1216   $28.12   $28.10   $28.10  
  Purchase #3 $82.81   5.673%   $87.5078   $87.51   $87.50   $87.50  
  Purchase #4 $226.68   5.464%   $239.0658   $239.07   $239.05   $239.10  
  Purchase #5 $40.70   10.763%   $45.0805   $45.08   $45.10   $45.10  
  Purchase #6 $147.71   9.804%   $162.1915   $162.19   $162.20   $162.20  
  Purchase #7 $94.99   5.143%   $99.8753   $99.88   $99.90   $99.90  
  7 purchase totals = $734.16       $789.8442   $789.85   $789.85   $789.90