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 $27.36   9.673%   $30.0065   $30.01   $30.00   $30.00  
  Purchase #2 $14.86   3.358%   $15.359   $15.36   $15.35   $15.40  
  Purchase #3 $136.27   5.528%   $143.803   $143.80   $143.80   $143.80  
  Purchase #4 $30.98   6.662%   $33.0439   $33.04   $33.05   $33.00  
  Purchase #5 $69.92   6.462%   $74.4382   $74.44   $74.45   $74.40  
  Purchase #6 $227.77   7.462%   $244.7662   $244.77   $244.75   $244.80  
  Purchase #7 $90.73   4.854%   $95.134   $95.13   $95.15   $95.10  
  7 purchase totals = $597.89       $636.5508   $636.55   $636.55   $636.50