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 $7.99   7.835%   $8.616   $8.62   $8.60   $8.60  
  Purchase #2 $69.38   11.608%   $77.4336   $77.43   $77.45   $77.40  
  Purchase #3 $92.56   3.874%   $96.1458   $96.15   $96.15   $96.10  
  Purchase #4 $9.84   4.850%   $10.3172   $10.32   $10.30   $10.30  
  Purchase #5 $155.72   5.855%   $164.8374   $164.84   $164.85   $164.80  
  Purchase #6 $55.68   3.597%   $57.6828   $57.68   $57.70   $57.70  
  Purchase #7 $87.44   10.649%   $96.7515   $96.75   $96.75   $96.80  
  7 purchase totals = $478.61       $511.7843   $511.79   $511.80   $511.70