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 $250.44   12.598%   $281.9904   $281.99   $282.00   $282.00  
  Purchase #2 $103.74   10.805%   $114.9491   $114.95   $114.95   $114.90  
  Purchase #3 $152.91   13.464%   $173.4978   $173.50   $173.50   $173.50  
  Purchase #4 $74.92   9.362%   $81.934   $81.93   $81.95   $81.90  
  Purchase #5 $61.95   13.728%   $70.4545   $70.45   $70.45   $70.50  
  Purchase #6 $220.81   11.312%   $245.788   $245.79   $245.80   $245.80  
  Purchase #7 $228.52   4.395%   $238.5635   $238.56   $238.55   $238.60  
  7 purchase totals = $1,093.29       $1,207.1773   $1,207.17   $1,207.20   $1,207.20