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 $34.41   12.351%   $38.66   $38.66   $38.65   $38.70  
  Purchase #2 $213.51   13.228%   $241.7531   $241.75   $241.75   $241.80  
  Purchase #3 $32.58   4.178%   $33.9412   $33.94   $33.95   $33.90  
  Purchase #4 $239.31   10.188%   $263.6909   $263.69   $263.70   $263.70  
  Purchase #5 $181.52   10.411%   $200.418   $200.42   $200.40   $200.40  
  Purchase #6 $58.87   6.398%   $62.6365   $62.64   $62.65   $62.60  
  Purchase #7 $100.54   8.103%   $108.6868   $108.69   $108.70   $108.70  
  7 purchase totals = $860.74       $949.7865   $949.79   $949.80   $949.80