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 $137.96   4.378%   $143.9999   $144.00   $144.00   $144.00  
  Purchase #2 $21.41   4.849%   $22.4482   $22.45   $22.45   $22.40  
  Purchase #3 $230.43   5.729%   $243.6313   $243.63   $243.65   $243.60  
  Purchase #4 $12.39   3.384%   $12.8093   $12.81   $12.80   $12.80  
  Purchase #5 $32.73   3.530%   $33.8854   $33.89   $33.90   $33.90  
  Purchase #6 $139.74   13.247%   $158.2514   $158.25   $158.25   $158.30  
  Purchase #7 $61.58   11.712%   $68.7922   $68.79   $68.80   $68.80  
  7 purchase totals = $636.24       $683.8177   $683.82   $683.85   $683.80