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 $76.73   4.234%   $79.9787   $79.98   $80.00   $80.00  
  Purchase #2 $7.41   12.140%   $8.3096   $8.31   $8.30   $8.30  
  Purchase #3 $37.66   5.857%   $39.8657   $39.87   $39.85   $39.90  
  Purchase #4 $68.87   12.849%   $77.7191   $77.72   $77.70   $77.70  
  Purchase #5 $142.81   4.178%   $148.7766   $148.78   $148.80   $148.80  
  Purchase #6 $224.51   6.160%   $238.3398   $238.34   $238.35   $238.30  
  Purchase #7 $73.57   5.241%   $77.4258   $77.43   $77.45   $77.40  
  7 purchase totals = $631.56       $670.4153   $670.43   $670.45   $670.40