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 $201.67   12.337%   $226.55   $226.55   $226.55   $226.60  
  Purchase #2 $32.86   6.524%   $35.0038   $35.00   $35.00   $35.00  
  Purchase #3 $220.56   10.789%   $244.3562   $244.36   $244.35   $244.40  
  Purchase #4 $148.63   3.158%   $153.3237   $153.32   $153.30   $153.30  
  Purchase #5 $18.54   12.273%   $20.8154   $20.82   $20.80   $20.80  
  Purchase #6 $113.48   6.603%   $120.9731   $120.97   $120.95   $121.00  
  Purchase #7 $175.60   9.720%   $192.6683   $192.67   $192.65   $192.70  
  7 purchase totals = $911.34       $993.6905   $993.69   $993.60   $993.80