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 $244.67   4.613%   $255.9566   $255.96   $255.95   $256.00  
  Purchase #2 $136.29   11.237%   $151.6049   $151.60   $151.60   $151.60  
  Purchase #3 $20.58   12.810%   $23.2163   $23.22   $23.20   $23.20  
  Purchase #4 $205.55   3.760%   $213.2787   $213.28   $213.30   $213.30  
  Purchase #5 $176.65   9.558%   $193.5342   $193.53   $193.55   $193.50  
  Purchase #6 $62.42   13.638%   $70.9328   $70.93   $70.95   $70.90  
  Purchase #7 $48.66   9.639%   $53.3503   $53.35   $53.35   $53.40  
  7 purchase totals = $894.82       $961.8738   $961.87   $961.90   $961.90