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 $241.83   6.762%   $258.1825   $258.18   $258.20   $258.20  
  Purchase #2 $72.54   7.514%   $77.9907   $77.99   $78.00   $78.00  
  Purchase #3 $185.33   11.377%   $206.415   $206.42   $206.40   $206.40  
  Purchase #4 $227.79   10.831%   $252.4619   $252.46   $252.45   $252.50  
  Purchase #5 $150.28   13.352%   $170.3454   $170.35   $170.35   $170.30  
  Purchase #6 $240.90   11.707%   $269.1022   $269.10   $269.10   $269.10  
  Purchase #7 $232.98   11.194%   $259.0598   $259.06   $259.05   $259.10  
  7 purchase totals = $1,351.65       $1,493.5575   $1,493.56   $1,493.55   $1,493.60