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 $30.30   5.179%   $31.8692   $31.87   $31.85   $31.90  
  Purchase #2 $28.62   9.871%   $31.4451   $31.45   $31.45   $31.40  
  Purchase #3 $115.63   12.731%   $130.3509   $130.35   $130.35   $130.40  
  Purchase #4 $226.59   9.414%   $247.9212   $247.92   $247.90   $247.90  
  Purchase #5 $140.74   13.217%   $159.3416   $159.34   $159.35   $159.30  
  Purchase #6 $176.81   10.172%   $194.7951   $194.80   $194.80   $194.80  
  Purchase #7 $215.25   8.235%   $232.9758   $232.98   $233.00   $233.00  
  7 purchase totals = $933.94       $1,028.6989   $1,028.71   $1,028.70   $1,028.70