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 $27.38   9.524%   $29.9877   $29.99   $30.00   $30.00  
  Purchase #2 $215.31   6.231%   $228.726   $228.73   $228.75   $228.70  
  Purchase #3 $129.70   9.131%   $141.5429   $141.54   $141.55   $141.50  
  Purchase #4 $12.87   11.418%   $14.3395   $14.34   $14.35   $14.30  
  Purchase #5 $70.46   5.671%   $74.4558   $74.46   $74.45   $74.50  
  Purchase #6 $235.29   5.772%   $248.8709   $248.87   $248.85   $248.90  
  Purchase #7 $96.78   11.672%   $108.0762   $108.08   $108.10   $108.10  
  7 purchase totals = $787.79       $845.9990   $846.01   $846.05   $846.00