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 $79.57   6.532%   $84.7675   $84.77   $84.75   $84.80  
  Purchase #2 $227.93   4.652%   $238.5333   $238.53   $238.55   $238.50  
  Purchase #3 $113.53   5.688%   $119.9876   $119.99   $120.00   $120.00  
  Purchase #4 $46.36   11.202%   $51.5532   $51.55   $51.55   $51.60  
  Purchase #5 $57.52   6.235%   $61.1064   $61.11   $61.10   $61.10  
  Purchase #6 $36.83   6.689%   $39.2936   $39.29   $39.30   $39.30  
  Purchase #7 $54.50   7.790%   $58.7456   $58.75   $58.75   $58.70  
  7 purchase totals = $616.24       $653.9872   $653.99   $654.00   $654.00