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 $250.85   4.866%   $263.0564   $263.06   $263.05   $263.10  
  Purchase #2 $203.30   12.351%   $228.4096   $228.41   $228.40   $228.40  
  Purchase #3 $62.84   3.708%   $65.1701   $65.17   $65.15   $65.20  
  Purchase #4 $99.34   8.819%   $108.1008   $108.10   $108.10   $108.10  
  Purchase #5 $188.72   5.381%   $198.875   $198.88   $198.90   $198.90  
  Purchase #6 $23.53   13.169%   $26.6287   $26.63   $26.65   $26.60  
  Purchase #7 $102.25   9.314%   $111.7736   $111.77   $111.75   $111.80  
  7 purchase totals = $930.83       $1,002.0142   $1,002.02   $1,002.00   $1,002.10