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 $69.52   13.282%   $78.7536   $78.75   $78.75   $78.80  
  Purchase #2 $233.35   10.470%   $257.7817   $257.78   $257.80   $257.80  
  Purchase #3 $102.76   12.742%   $115.8537   $115.85   $115.85   $115.90  
  Purchase #4 $164.51   11.393%   $183.2526   $183.25   $183.25   $183.30  
  Purchase #5 $29.50   11.295%   $32.832   $32.83   $32.85   $32.80  
  Purchase #6 $66.84   11.260%   $74.3662   $74.37   $74.35   $74.40  
  Purchase #7 $228.82   9.143%   $249.741   $249.74   $249.75   $249.70  
  7 purchase totals = $895.30       $992.5808   $992.57   $992.60   $992.70