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 $168.64   13.737%   $191.8061   $191.81   $191.80   $191.80  
  Purchase #2 $231.41   11.281%   $257.5154   $257.52   $257.50   $257.50  
  Purchase #3 $32.27   11.676%   $36.0378   $36.04   $36.05   $36.00  
  Purchase #4 $141.89   12.221%   $159.2304   $159.23   $159.25   $159.20  
  Purchase #5 $190.65   12.355%   $214.2048   $214.20   $214.20   $214.20  
  Purchase #6 $21.58   10.586%   $23.8645   $23.86   $23.85   $23.90  
  Purchase #7 $169.79   8.188%   $183.6924   $183.69   $183.70   $183.70  
  7 purchase totals = $956.23       $1,066.3514   $1,066.35   $1,066.35   $1,066.30