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 $129.38   7.858%   $139.5467   $139.55   $139.55   $139.50  
  Purchase #2 $140.63   4.587%   $147.0807   $147.08   $147.10   $147.10  
  Purchase #3 $134.73   11.377%   $150.0582   $150.06   $150.05   $150.10  
  Purchase #4 $39.57   12.312%   $44.4419   $44.44   $44.45   $44.40  
  Purchase #5 $110.55   4.405%   $115.4197   $115.42   $115.40   $115.40  
  Purchase #6 $79.61   11.131%   $88.4714   $88.47   $88.45   $88.50  
  Purchase #7 $30.89   6.574%   $32.9207   $32.92   $32.90   $32.90  
  7 purchase totals = $665.36       $717.9393   $717.94   $717.90   $717.90