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 $18.42   10.163%   $20.292   $20.29   $20.30   $20.30  
  Purchase #2 $33.47   4.787%   $35.0722   $35.07   $35.05   $35.10  
  Purchase #3 $18.94   12.525%   $21.3122   $21.31   $21.30   $21.30  
  Purchase #4 $48.77   9.526%   $53.4158   $53.42   $53.40   $53.40  
  Purchase #5 $181.27   9.140%   $197.8381   $197.84   $197.85   $197.80  
  Purchase #6 $244.69   3.360%   $252.9116   $252.91   $252.90   $252.90  
  Purchase #7 $55.91   9.747%   $61.3595   $61.36   $61.35   $61.40  
  7 purchase totals = $601.47       $642.2014   $642.20   $642.15   $642.20