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 $80.54   4.583%   $84.2311   $84.23   $84.25   $84.20  
  Purchase #2 $192.80   11.490%   $214.9527   $214.95   $214.95   $215.00  
  Purchase #3 $87.48   8.479%   $94.8974   $94.90   $94.90   $94.90  
  Purchase #4 $216.28   8.526%   $234.72   $234.72   $234.70   $234.70  
  Purchase #5 $80.60   6.874%   $86.1404   $86.14   $86.15   $86.10  
  Purchase #6 $51.84   6.760%   $55.3444   $55.34   $55.35   $55.30  
  Purchase #7 $111.88   8.571%   $121.4692   $121.47   $121.45   $121.50  
  7 purchase totals = $821.42       $891.7552   $891.75   $891.75   $891.70