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 $236.85   10.306%   $261.2598   $261.26   $261.25   $261.30  
  Purchase #2 $138.86   11.843%   $155.3052   $155.31   $155.30   $155.30  
  Purchase #3 $43.86   5.174%   $46.1293   $46.13   $46.15   $46.10  
  Purchase #4 $197.34   11.131%   $219.3059   $219.31   $219.30   $219.30  
  Purchase #5 $209.64   11.586%   $233.9289   $233.93   $233.95   $233.90  
  Purchase #6 $18.82   7.361%   $20.2053   $20.21   $20.20   $20.20  
  Purchase #7 $65.58   4.636%   $68.6203   $68.62   $68.60   $68.60  
  7 purchase totals = $910.95       $1,004.7547   $1,004.77   $1,004.75   $1,004.70