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 $138.31   3.480%   $143.1232   $143.12   $143.10   $143.10  
  Purchase #2 $81.36   10.439%   $89.8532   $89.85   $89.85   $89.90  
  Purchase #3 $238.38   4.182%   $248.3491   $248.35   $248.35   $248.30  
  Purchase #4 $45.61   5.371%   $48.0597   $48.06   $48.05   $48.10  
  Purchase #5 $16.54   11.519%   $18.4452   $18.45   $18.45   $18.40  
  Purchase #6 $172.32   6.252%   $183.0934   $183.09   $183.10   $183.10  
  Purchase #7 $53.71   9.224%   $58.6642   $58.66   $58.65   $58.70  
  7 purchase totals = $746.23       $789.5880   $789.58   $789.55   $789.60