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 $199.32   5.800%   $210.8806   $210.88   $210.90   $210.90  
  Purchase #2 $75.30   6.240%   $79.9987   $80.00   $80.00   $80.00  
  Purchase #3 $13.61   13.253%   $15.4137   $15.41   $15.40   $15.40  
  Purchase #4 $15.50   8.681%   $16.8456   $16.85   $16.85   $16.80  
  Purchase #5 $174.86   4.266%   $182.3195   $182.32   $182.30   $182.30  
  Purchase #6 $99.79   8.358%   $108.1304   $108.13   $108.15   $108.10  
  Purchase #7 $227.65   12.703%   $256.5684   $256.57   $256.55   $256.60  
  7 purchase totals = $806.03       $870.1569   $870.16   $870.15   $870.10