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 $28.48   8.181%   $30.8099   $30.81   $30.80   $30.80  
  Purchase #2 $193.94   8.323%   $210.0816   $210.08   $210.10   $210.10  
  Purchase #3 $199.89   6.296%   $212.4751   $212.48   $212.50   $212.50  
  Purchase #4 $215.94   4.239%   $225.0937   $225.09   $225.10   $225.10  
  Purchase #5 $135.99   12.346%   $152.7793   $152.78   $152.80   $152.80  
  Purchase #6 $209.47   10.661%   $231.8016   $231.80   $231.80   $231.80  
  Purchase #7 $21.59   8.397%   $23.4029   $23.40   $23.40   $23.40  
  7 purchase totals = $1,005.30       $1,086.4441   $1,086.44   $1,086.50   $1,086.50