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 $59.56   8.131%   $64.4028   $64.40   $64.40   $64.40  
  Purchase #2 $147.68   12.849%   $166.6554   $166.66   $166.65   $166.70  
  Purchase #3 $55.89   10.404%   $61.7048   $61.70   $61.70   $61.70  
  Purchase #4 $131.37   8.505%   $142.543   $142.54   $142.55   $142.50  
  Purchase #5 $71.77   5.381%   $75.6319   $75.63   $75.65   $75.60  
  Purchase #6 $43.36   13.273%   $49.1152   $49.12   $49.10   $49.10  
  Purchase #7 $5.46   12.170%   $6.1245   $6.12   $6.10   $6.10  
  7 purchase totals = $515.09       $566.1776   $566.17   $566.15   $566.10