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 $29.94   13.496%   $33.9807   $33.98   $34.00   $34.00  
  Purchase #2 $64.42   11.864%   $72.0628   $72.06   $72.05   $72.10  
  Purchase #3 $55.43   5.830%   $58.6616   $58.66   $58.65   $58.70  
  Purchase #4 $208.33   8.189%   $225.3901   $225.39   $225.40   $225.40  
  Purchase #5 $20.39   12.644%   $22.9681   $22.97   $22.95   $23.00  
  Purchase #6 $19.48   3.582%   $20.1778   $20.18   $20.20   $20.20  
  Purchase #7 $186.87   7.561%   $200.9992   $201.00   $201.00   $201.00  
  7 purchase totals = $584.86       $634.2403   $634.24   $634.25   $634.40