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 $220.47   4.503%   $230.3978   $230.40   $230.40   $230.40  
  Purchase #2 $102.32   10.619%   $113.1854   $113.19   $113.20   $113.20  
  Purchase #3 $210.56   3.626%   $218.1949   $218.19   $218.20   $218.20  
  Purchase #4 $227.26   10.244%   $250.5405   $250.54   $250.55   $250.50  
  Purchase #5 $241.52   3.810%   $250.7219   $250.72   $250.70   $250.70  
  Purchase #6 $42.57   4.659%   $44.5533   $44.55   $44.55   $44.60  
  Purchase #7 $178.71   13.253%   $202.3944   $202.39   $202.40   $202.40  
  7 purchase totals = $1,223.41       $1,309.9882   $1,309.98   $1,310.00   $1,310.00