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 $164.76   3.309%   $170.2119   $170.21   $170.20   $170.20  
  Purchase #2 $238.97   7.488%   $256.8641   $256.86   $256.85   $256.90  
  Purchase #3 $193.54   8.842%   $210.6528   $210.65   $210.65   $210.70  
  Purchase #4 $219.82   8.840%   $239.2521   $239.25   $239.25   $239.30  
  Purchase #5 $193.91   12.601%   $218.3446   $218.34   $218.35   $218.30  
  Purchase #6 $104.25   4.463%   $108.9027   $108.90   $108.90   $108.90  
  Purchase #7 $118.31   12.717%   $133.3555   $133.36   $133.35   $133.40  
  7 purchase totals = $1,233.56       $1,337.5837   $1,337.57   $1,337.55   $1,337.70