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 $21.54   5.715%   $22.771   $22.77   $22.75   $22.80  
  Purchase #2 $222.82   3.320%   $230.2176   $230.22   $230.20   $230.20  
  Purchase #3 $104.75   3.176%   $108.0769   $108.08   $108.10   $108.10  
  Purchase #4 $35.83   11.593%   $39.9838   $39.98   $40.00   $40.00  
  Purchase #5 $22.59   4.867%   $23.6895   $23.69   $23.70   $23.70  
  Purchase #6 $17.35   4.699%   $18.1653   $18.17   $18.15   $18.20  
  Purchase #7 $205.64   10.760%   $227.7669   $227.77   $227.75   $227.80  
  7 purchase totals = $630.52       $670.6710   $670.68   $670.65   $670.80