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 $30.84   12.155%   $34.5886   $34.59   $34.60   $34.60  
  Purchase #2 $200.89   10.630%   $222.2446   $222.24   $222.25   $222.20  
  Purchase #3 $243.32   5.769%   $257.3571   $257.36   $257.35   $257.40  
  Purchase #4 $163.73   13.511%   $185.8516   $185.85   $185.85   $185.90  
  Purchase #5 $191.78   5.360%   $202.0594   $202.06   $202.05   $202.10  
  Purchase #6 $201.51   13.357%   $228.4257   $228.43   $228.45   $228.40  
  Purchase #7 $61.92   11.790%   $69.2204   $69.22   $69.20   $69.20  
  7 purchase totals = $1,093.99       $1,199.7474   $1,199.75   $1,199.75   $1,199.80