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 $126.36   4.339%   $131.8428   $131.84   $131.85   $131.80  
  Purchase #2 $30.29   3.191%   $31.2566   $31.26   $31.25   $31.30  
  Purchase #3 $63.71   10.498%   $70.3983   $70.40   $70.40   $70.40  
  Purchase #4 $13.43   12.192%   $15.0674   $15.07   $15.05   $15.10  
  Purchase #5 $81.35   3.224%   $83.9727   $83.97   $83.95   $84.00  
  Purchase #6 $75.93   6.339%   $80.7432   $80.74   $80.75   $80.70  
  Purchase #7 $201.89   12.667%   $227.4634   $227.46   $227.45   $227.50  
  7 purchase totals = $592.96       $640.7444   $640.74   $640.70   $640.80