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 $21.32   9.235%   $23.2889   $23.29   $23.30   $23.30  
  Purchase #2 $27.78   3.700%   $28.8079   $28.81   $28.80   $28.80  
  Purchase #3 $210.90   3.724%   $218.7539   $218.75   $218.75   $218.80  
  Purchase #4 $197.25   9.411%   $215.8132   $215.81   $215.80   $215.80  
  Purchase #5 $128.31   6.168%   $136.2242   $136.22   $136.20   $136.20  
  Purchase #6 $168.97   7.250%   $181.2203   $181.22   $181.20   $181.20  
  Purchase #7 $227.68   8.457%   $246.9349   $246.93   $246.95   $246.90  
  7 purchase totals = $982.21       $1,051.0433   $1,051.03   $1,051.00   $1,051.00