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 $102.54   4.685%   $107.344   $107.34   $107.35   $107.30  
  Purchase #2 $85.49   13.810%   $97.2962   $97.30   $97.30   $97.30  
  Purchase #3 $36.44   9.704%   $39.9761   $39.98   $40.00   $40.00  
  Purchase #4 $102.84   12.378%   $115.5695   $115.57   $115.55   $115.60  
  Purchase #5 $15.88   5.255%   $16.7145   $16.71   $16.70   $16.70  
  Purchase #6 $110.53   11.424%   $123.1569   $123.16   $123.15   $123.20  
  Purchase #7 $130.90   7.198%   $140.3222   $140.32   $140.30   $140.30  
  7 purchase totals = $584.62       $640.3794   $640.38   $640.35   $640.40