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 $40.42   4.292%   $42.1548   $42.15   $42.15   $42.20  
  Purchase #2 $247.66   13.225%   $280.413   $280.41   $280.40   $280.40  
  Purchase #3 $110.85   4.421%   $115.7507   $115.75   $115.75   $115.80  
  Purchase #4 $18.96   11.408%   $21.123   $21.12   $21.10   $21.10  
  Purchase #5 $107.80   7.350%   $115.7233   $115.72   $115.70   $115.70  
  Purchase #6 $149.53   13.481%   $169.6881   $169.69   $169.70   $169.70  
  Purchase #7 $97.43   8.141%   $105.3618   $105.36   $105.35   $105.40  
  7 purchase totals = $772.65       $850.2147   $850.20   $850.15   $850.30