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 $111.57   13.191%   $126.2872   $126.29   $126.30   $126.30  
  Purchase #2 $242.37   3.157%   $250.0216   $250.02   $250.00   $250.00  
  Purchase #3 $185.37   4.194%   $193.1444   $193.14   $193.15   $193.10  
  Purchase #4 $12.56   13.103%   $14.2057   $14.21   $14.20   $14.20  
  Purchase #5 $171.58   7.140%   $183.8308   $183.83   $183.85   $183.80  
  Purchase #6 $162.58   12.206%   $182.4245   $182.42   $182.40   $182.40  
  Purchase #7 $31.32   12.811%   $35.3324   $35.33   $35.35   $35.30  
  7 purchase totals = $917.35       $985.2466   $985.24   $985.25   $985.10