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 $27.85   13.798%   $31.6927   $31.69   $31.70   $31.70  
  Purchase #2 $34.67   9.415%   $37.9342   $37.93   $37.95   $37.90  
  Purchase #3 $229.58   12.609%   $258.5277   $258.53   $258.55   $258.50  
  Purchase #4 $108.41   8.860%   $118.0151   $118.02   $118.00   $118.00  
  Purchase #5 $75.29   8.835%   $81.9419   $81.94   $81.95   $81.90  
  Purchase #6 $140.44   13.640%   $159.596   $159.60   $159.60   $159.60  
  Purchase #7 $155.89   13.748%   $177.3218   $177.32   $177.30   $177.30  
  7 purchase totals = $772.13       $865.0294   $865.03   $865.05   $864.90