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 $248.72   13.688%   $282.7648   $282.76   $282.75   $282.80  
  Purchase #2 $56.56   11.384%   $62.9988   $63.00   $63.00   $63.00  
  Purchase #3 $239.66   13.255%   $271.4269   $271.43   $271.45   $271.40  
  Purchase #4 $21.61   6.187%   $22.947   $22.95   $22.95   $22.90  
  Purchase #5 $57.47   13.620%   $65.2974   $65.30   $65.30   $65.30  
  Purchase #6 $54.64   12.626%   $61.5388   $61.54   $61.55   $61.50  
  Purchase #7 $181.39   12.457%   $203.9858   $203.99   $204.00   $204.00  
  7 purchase totals = $860.05       $970.9595   $970.97   $971.00   $970.90