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 $29.57   6.388%   $31.4589   $31.46   $31.45   $31.50  
  Purchase #2 $27.81   6.361%   $29.579   $29.58   $29.60   $29.60  
  Purchase #3 $101.77   6.281%   $108.1622   $108.16   $108.15   $108.20  
  Purchase #4 $239.32   8.869%   $260.5453   $260.55   $260.55   $260.50  
  Purchase #5 $49.99   10.479%   $55.2285   $55.23   $55.25   $55.20  
  Purchase #6 $5.90   7.135%   $6.321   $6.32   $6.30   $6.30  
  Purchase #7 $124.78   3.732%   $129.4368   $129.44   $129.45   $129.40  
  7 purchase totals = $579.14       $620.7317   $620.74   $620.75   $620.70