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 $83.77   7.428%   $89.9924   $89.99   $90.00   $90.00  
  Purchase #2 $20.85   13.142%   $23.5901   $23.59   $23.60   $23.60  
  Purchase #3 $145.33   9.564%   $159.2294   $159.23   $159.25   $159.20  
  Purchase #4 $231.35   11.154%   $257.1548   $257.15   $257.15   $257.20  
  Purchase #5 $208.47   10.526%   $230.4136   $230.41   $230.40   $230.40  
  Purchase #6 $44.84   7.572%   $48.2353   $48.24   $48.25   $48.20  
  Purchase #7 $17.53   11.116%   $19.4786   $19.48   $19.50   $19.50  
  7 purchase totals = $752.14       $828.0942   $828.09   $828.15   $828.10