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 $12.59   10.216%   $13.8762   $13.88   $13.90   $13.90  
  Purchase #2 $239.39   13.108%   $270.7692   $270.77   $270.75   $270.80  
  Purchase #3 $42.33   6.335%   $45.0116   $45.01   $45.00   $45.00  
  Purchase #4 $106.54   13.358%   $120.7716   $120.77   $120.75   $120.80  
  Purchase #5 $128.44   6.254%   $136.4726   $136.47   $136.45   $136.50  
  Purchase #6 $32.66   7.795%   $35.2058   $35.21   $35.20   $35.20  
  Purchase #7 $122.33   13.690%   $139.077   $139.08   $139.10   $139.10  
  7 purchase totals = $684.28       $761.1840   $761.19   $761.15   $761.30