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 $53.98   3.666%   $55.9589   $55.96   $55.95   $56.00  
  Purchase #2 $118.38   5.509%   $124.9016   $124.90   $124.90   $124.90  
  Purchase #3 $99.68   5.623%   $105.285   $105.29   $105.30   $105.30  
  Purchase #4 $39.58   11.818%   $44.2576   $44.26   $44.25   $44.30  
  Purchase #5 $46.26   6.577%   $49.3025   $49.30   $49.30   $49.30  
  Purchase #6 $144.77   10.414%   $159.8463   $159.85   $159.85   $159.80  
  Purchase #7 $230.48   12.526%   $259.3499   $259.35   $259.35   $259.30  
  7 purchase totals = $733.13       $798.9018   $798.91   $798.90   $798.90