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 $19.61   8.320%   $21.2416   $21.24   $21.25   $21.20  
  Purchase #2 $115.92   12.844%   $130.8088   $130.81   $130.80   $130.80  
  Purchase #3 $151.55   6.239%   $161.0052   $161.01   $161.00   $161.00  
  Purchase #4 $103.25   7.396%   $110.8864   $110.89   $110.90   $110.90  
  Purchase #5 $96.35   4.514%   $100.6992   $100.70   $100.70   $100.70  
  Purchase #6 $79.99   11.628%   $89.2912   $89.29   $89.30   $89.30  
  Purchase #7 $158.83   3.158%   $163.8459   $163.85   $163.85   $163.80  
  7 purchase totals = $725.50       $777.7783   $777.79   $777.80   $777.70