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 $48.83   13.112%   $55.2326   $55.23   $55.25   $55.20  
  Purchase #2 $54.92   6.261%   $58.3585   $58.36   $58.35   $58.40  
  Purchase #3 $147.41   8.827%   $160.4219   $160.42   $160.40   $160.40  
  Purchase #4 $191.96   11.646%   $214.3157   $214.32   $214.30   $214.30  
  Purchase #5 $156.69   9.339%   $171.3233   $171.32   $171.30   $171.30  
  Purchase #6 $30.63   7.166%   $32.8249   $32.82   $32.80   $32.80  
  Purchase #7 $53.42   7.513%   $57.4334   $57.43   $57.45   $57.40  
  7 purchase totals = $683.86       $749.9103   $749.90   $749.85   $749.80