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 $246.63   5.399%   $259.9456   $259.95   $259.95   $259.90  
  Purchase #2 $52.86   12.232%   $59.3258   $59.33   $59.35   $59.30  
  Purchase #3 $152.84   7.370%   $164.1043   $164.10   $164.10   $164.10  
  Purchase #4 $152.67   9.551%   $167.2515   $167.25   $167.25   $167.30  
  Purchase #5 $7.74   3.598%   $8.0185   $8.02   $8.00   $8.00  
  Purchase #6 $126.87   10.218%   $139.8336   $139.83   $139.85   $139.80  
  Purchase #7 $32.48   3.167%   $33.5086   $33.51   $33.50   $33.50  
  7 purchase totals = $772.09       $831.9879   $831.99   $832.00   $831.90