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 $102.72   6.355%   $109.2479   $109.25   $109.25   $109.20  
  Purchase #2 $30.44   7.478%   $32.7163   $32.72   $32.70   $32.70  
  Purchase #3 $210.54   13.201%   $238.3334   $238.33   $238.35   $238.30  
  Purchase #4 $99.61   5.319%   $104.9083   $104.91   $104.90   $104.90  
  Purchase #5 $178.84   7.662%   $192.5427   $192.54   $192.55   $192.50  
  Purchase #6 $196.57   10.469%   $217.1489   $217.15   $217.15   $217.10  
  Purchase #7 $25.31   4.319%   $26.4031   $26.40   $26.40   $26.40  
  7 purchase totals = $844.03       $921.3006   $921.30   $921.30   $921.10