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 $16.83   11.519%   $18.7686   $18.77   $18.75   $18.80  
  Purchase #2 $13.42   8.852%   $14.6079   $14.61   $14.60   $14.60  
  Purchase #3 $81.37   8.217%   $88.0562   $88.06   $88.05   $88.10  
  Purchase #4 $185.56   4.664%   $194.2145   $194.21   $194.20   $194.20  
  Purchase #5 $45.33   13.199%   $51.3131   $51.31   $51.30   $51.30  
  Purchase #6 $78.44   4.792%   $82.1988   $82.20   $82.20   $82.20  
  Purchase #7 $58.67   7.849%   $63.275   $63.28   $63.30   $63.30  
  7 purchase totals = $479.62       $512.4341   $512.44   $512.40   $512.50