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 $70.92   13.570%   $80.5438   $80.54   $80.55   $80.50  
  Purchase #2 $205.60   10.325%   $226.8282   $226.83   $226.85   $226.80  
  Purchase #3 $130.32   12.771%   $146.9632   $146.96   $146.95   $147.00  
  Purchase #4 $178.46   4.241%   $186.0285   $186.03   $186.05   $186.00  
  Purchase #5 $43.46   13.453%   $49.3067   $49.31   $49.30   $49.30  
  Purchase #6 $56.25   4.266%   $58.6496   $58.65   $58.65   $58.60  
  Purchase #7 $178.26   13.841%   $202.933   $202.93   $202.95   $202.90  
  7 purchase totals = $863.27       $951.2530   $951.25   $951.30   $951.10