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 $249.76   13.665%   $283.8897   $283.89   $283.90   $283.90  
  Purchase #2 $174.54   9.686%   $191.4459   $191.45   $191.45   $191.40  
  Purchase #3 $193.75   10.858%   $214.7874   $214.79   $214.80   $214.80  
  Purchase #4 $141.87   7.149%   $152.0123   $152.01   $152.00   $152.00  
  Purchase #5 $159.41   13.579%   $181.0563   $181.06   $181.05   $181.10  
  Purchase #6 $178.95   5.591%   $188.9551   $188.96   $188.95   $189.00  
  Purchase #7 $204.27   4.858%   $214.1934   $214.19   $214.20   $214.20  
  7 purchase totals = $1,302.55       $1,426.3401   $1,426.35   $1,426.35   $1,426.40