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 $81.75   3.121%   $84.3014   $84.30   $84.30   $84.30  
  Purchase #2 $114.83   8.542%   $124.6388   $124.64   $124.65   $124.60  
  Purchase #3 $54.77   9.283%   $59.8543   $59.85   $59.85   $59.90  
  Purchase #4 $62.50   6.848%   $66.78   $66.78   $66.80   $66.80  
  Purchase #5 $108.63   9.581%   $119.0378   $119.04   $119.05   $119.00  
  Purchase #6 $18.48   12.802%   $20.8458   $20.85   $20.85   $20.80  
  Purchase #7 $114.59   13.651%   $130.2327   $130.23   $130.25   $130.20  
  7 purchase totals = $555.55       $605.6908   $605.69   $605.75   $605.60