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 $214.31   10.310%   $236.4054   $236.41   $236.40   $236.40  
  Purchase #2 $37.64   5.393%   $39.6699   $39.67   $39.65   $39.70  
  Purchase #3 $60.28   6.694%   $64.3151   $64.32   $64.30   $64.30  
  Purchase #4 $165.43   11.209%   $183.973   $183.97   $183.95   $184.00  
  Purchase #5 $51.83   7.801%   $55.8733   $55.87   $55.85   $55.90  
  Purchase #6 $193.84   10.636%   $214.4568   $214.46   $214.45   $214.50  
  Purchase #7 $14.95   5.546%   $15.7791   $15.78   $15.80   $15.80  
  7 purchase totals = $738.28       $810.4726   $810.48   $810.40   $810.60