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 $109.32   6.480%   $116.4039   $116.40   $116.40   $116.40  
  Purchase #2 $14.64   3.121%   $15.0969   $15.10   $15.10   $15.10  
  Purchase #3 $26.96   5.510%   $28.4455   $28.45   $28.45   $28.40  
  Purchase #4 $208.43   11.174%   $231.72   $231.72   $231.70   $231.70  
  Purchase #5 $28.43   10.107%   $31.3034   $31.30   $31.30   $31.30  
  Purchase #6 $75.82   5.184%   $79.7505   $79.75   $79.75   $79.80  
  Purchase #7 $13.73   13.480%   $15.5808   $15.58   $15.60   $15.60  
  7 purchase totals = $477.33       $518.3010   $518.30   $518.30   $518.30