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 $16.40   8.605%   $17.8112   $17.81   $17.80   $17.80  
  Purchase #2 $33.31   13.850%   $37.9234   $37.92   $37.90   $37.90  
  Purchase #3 $198.53   10.444%   $219.2645   $219.26   $219.25   $219.30  
  Purchase #4 $81.90   5.406%   $86.3275   $86.33   $86.35   $86.30  
  Purchase #5 $161.67   7.684%   $174.0927   $174.09   $174.10   $174.10  
  Purchase #6 $20.41   13.230%   $23.1102   $23.11   $23.10   $23.10  
  Purchase #7 $209.64   5.675%   $221.5371   $221.54   $221.55   $221.50  
  7 purchase totals = $721.86       $780.0666   $780.06   $780.05   $780.00