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 $9.34   11.510%   $10.415   $10.42   $10.40   $10.40  
  Purchase #2 $149.51   4.236%   $155.8432   $155.84   $155.85   $155.80  
  Purchase #3 $198.40   8.351%   $214.9684   $214.97   $214.95   $215.00  
  Purchase #4 $193.28   12.301%   $217.0554   $217.06   $217.05   $217.10  
  Purchase #5 $224.31   6.805%   $239.5743   $239.57   $239.55   $239.60  
  Purchase #6 $218.35   11.599%   $243.6764   $243.68   $243.70   $243.70  
  Purchase #7 $201.62   3.581%   $208.84   $208.84   $208.85   $208.80  
  7 purchase totals = $1,194.81       $1,290.3727   $1,290.38   $1,290.35   $1,290.40