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 $37.65   7.726%   $40.5588   $40.56   $40.55   $40.60  
  Purchase #2 $85.39   12.165%   $95.7777   $95.78   $95.80   $95.80  
  Purchase #3 $203.65   5.328%   $214.5005   $214.50   $214.50   $214.50  
  Purchase #4 $122.71   13.618%   $139.4206   $139.42   $139.40   $139.40  
  Purchase #5 $21.53   5.348%   $22.6814   $22.68   $22.70   $22.70  
  Purchase #6 $209.68   5.397%   $220.9964   $221.00   $221.00   $221.00  
  Purchase #7 $100.46   4.606%   $105.0872   $105.09   $105.10   $105.10  
  7 purchase totals = $781.07       $839.0226   $839.03   $839.05   $839.10