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 $250.72   6.519%   $267.0644   $267.06   $267.05   $267.10  
  Purchase #2 $15.93   11.224%   $17.718   $17.72   $17.70   $17.70  
  Purchase #3 $217.65   8.214%   $235.5278   $235.53   $235.55   $235.50  
  Purchase #4 $125.80   6.645%   $134.1594   $134.16   $134.15   $134.20  
  Purchase #5 $238.59   6.843%   $254.9167   $254.92   $254.90   $254.90  
  Purchase #6 $102.68   11.255%   $114.2366   $114.24   $114.25   $114.20  
  Purchase #7 $201.58   8.747%   $219.2122   $219.21   $219.20   $219.20  
  7 purchase totals = $1,152.95       $1,242.8351   $1,242.84   $1,242.80   $1,242.80