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 $170.85   10.400%   $188.6184   $188.62   $188.60   $188.60  
  Purchase #2 $170.30   8.149%   $184.1777   $184.18   $184.20   $184.20  
  Purchase #3 $196.57   12.460%   $221.0626   $221.06   $221.05   $221.10  
  Purchase #4 $226.88   10.490%   $250.6797   $250.68   $250.70   $250.70  
  Purchase #5 $206.54   10.562%   $228.3548   $228.35   $228.35   $228.40  
  Purchase #6 $92.91   12.573%   $104.5916   $104.59   $104.60   $104.60  
  Purchase #7 $38.25   6.374%   $40.6881   $40.69   $40.70   $40.70  
  7 purchase totals = $1,102.30       $1,218.1729   $1,218.17   $1,218.20   $1,218.30