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 $249.80   9.581%   $273.7333   $273.73   $273.75   $273.70  
  Purchase #2 $159.42   12.322%   $179.0637   $179.06   $179.05   $179.10  
  Purchase #3 $165.90   5.663%   $175.2949   $175.29   $175.30   $175.30  
  Purchase #4 $73.32   9.360%   $80.1828   $80.18   $80.20   $80.20  
  Purchase #5 $104.46   7.791%   $112.5985   $112.60   $112.60   $112.60  
  Purchase #6 $79.83   8.159%   $86.3433   $86.34   $86.35   $86.30  
  Purchase #7 $236.43   13.526%   $268.4095   $268.41   $268.40   $268.40  
  7 purchase totals = $1,069.16       $1,175.6260   $1,175.61   $1,175.65   $1,175.60