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 $44.77   13.512%   $50.8193   $50.82   $50.80   $50.80  
  Purchase #2 $54.95   11.136%   $61.0692   $61.07   $61.05   $61.10  
  Purchase #3 $61.99   7.652%   $66.7335   $66.73   $66.75   $66.70  
  Purchase #4 $128.83   3.428%   $133.2463   $133.25   $133.25   $133.20  
  Purchase #5 $243.89   5.764%   $257.9478   $257.95   $257.95   $257.90  
  Purchase #6 $81.42   6.446%   $86.6683   $86.67   $86.65   $86.70  
  Purchase #7 $208.79   7.440%   $224.324   $224.32   $224.30   $224.30  
  7 purchase totals = $824.64       $880.8084   $880.81   $880.75   $880.70