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 $18.99   7.865%   $20.4836   $20.48   $20.50   $20.50  
  Purchase #2 $199.50   12.811%   $225.0579   $225.06   $225.05   $225.10  
  Purchase #3 $54.46   7.321%   $58.447   $58.45   $58.45   $58.40  
  Purchase #4 $142.57   7.633%   $153.4524   $153.45   $153.45   $153.50  
  Purchase #5 $72.86   8.149%   $78.7974   $78.80   $78.80   $78.80  
  Purchase #6 $76.93   13.375%   $87.2194   $87.22   $87.20   $87.20  
  Purchase #7 $160.92   3.851%   $167.117   $167.12   $167.10   $167.10  
  7 purchase totals = $726.23       $790.5747   $790.58   $790.55   $790.60