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 $228.50   12.156%   $256.2765   $256.28   $256.30   $256.30  
  Purchase #2 $96.32   11.216%   $107.1233   $107.12   $107.10   $107.10  
  Purchase #3 $152.68   9.738%   $167.548   $167.55   $167.55   $167.50  
  Purchase #4 $223.33   11.468%   $248.9415   $248.94   $248.95   $248.90  
  Purchase #5 $19.76   11.713%   $22.0745   $22.07   $22.05   $22.10  
  Purchase #6 $40.93   10.518%   $45.235   $45.24   $45.25   $45.20  
  Purchase #7 $124.74   9.644%   $136.7699   $136.77   $136.75   $136.80  
  7 purchase totals = $886.26       $983.9687   $983.97   $983.95   $983.90