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 $75.25   12.477%   $84.6389   $84.64   $84.65   $84.60  
  Purchase #2 $20.97   10.789%   $23.2325   $23.23   $23.25   $23.20  
  Purchase #3 $238.35   3.844%   $247.5122   $247.51   $247.50   $247.50  
  Purchase #4 $58.94   9.722%   $64.6701   $64.67   $64.65   $64.70  
  Purchase #5 $153.59   13.560%   $174.4168   $174.42   $174.40   $174.40  
  Purchase #6 $200.40   12.103%   $224.6544   $224.65   $224.65   $224.70  
  Purchase #7 $59.42   13.791%   $67.6146   $67.61   $67.60   $67.60  
  7 purchase totals = $806.92       $886.7395   $886.73   $886.70   $886.70