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 $20.70   5.768%   $21.894   $21.89   $21.90   $21.90  
  Purchase #2 $129.87   5.180%   $136.5973   $136.60   $136.60   $136.60  
  Purchase #3 $104.67   13.497%   $118.7973   $118.80   $118.80   $118.80  
  Purchase #4 $122.38   7.699%   $131.802   $131.80   $131.80   $131.80  
  Purchase #5 $18.45   8.283%   $19.9782   $19.98   $20.00   $20.00  
  Purchase #6 $124.60   7.351%   $133.7593   $133.76   $133.75   $133.80  
  Purchase #7 $64.57   5.500%   $68.1214   $68.12   $68.10   $68.10  
  7 purchase totals = $585.24       $630.9495   $630.95   $630.95   $631.00