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 $18.61   12.750%   $20.9828   $20.98   $21.00   $21.00  
  Purchase #2 $47.57   9.765%   $52.2152   $52.22   $52.20   $52.20  
  Purchase #3 $92.25   4.199%   $96.1236   $96.12   $96.10   $96.10  
  Purchase #4 $133.80   10.629%   $148.0216   $148.02   $148.00   $148.00  
  Purchase #5 $226.58   9.480%   $248.0598   $248.06   $248.05   $248.10  
  Purchase #6 $229.45   3.571%   $237.6437   $237.64   $237.65   $237.60  
  Purchase #7 $19.26   13.224%   $21.8069   $21.81   $21.80   $21.80  
  7 purchase totals = $767.52       $824.8536   $824.85   $824.80   $824.80