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 $59.87   13.755%   $68.1051   $68.11   $68.10   $68.10  
  Purchase #2 $109.79   7.134%   $117.6224   $117.62   $117.60   $117.60  
  Purchase #3 $15.25   12.775%   $17.1982   $17.20   $17.20   $17.20  
  Purchase #4 $31.35   6.698%   $33.4498   $33.45   $33.45   $33.40  
  Purchase #5 $130.95   4.287%   $136.5638   $136.56   $136.55   $136.60  
  Purchase #6 $177.49   9.201%   $193.8209   $193.82   $193.80   $193.80  
  Purchase #7 $96.42   3.316%   $99.6173   $99.62   $99.60   $99.60  
  7 purchase totals = $621.12       $666.3775   $666.38   $666.30   $666.30