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 $92.73   4.188%   $96.6135   $96.61   $96.60   $96.60  
  Purchase #2 $103.86   7.184%   $111.3213   $111.32   $111.30   $111.30  
  Purchase #3 $112.84   7.839%   $121.6855   $121.69   $121.70   $121.70  
  Purchase #4 $129.74   3.313%   $134.0383   $134.04   $134.05   $134.00  
  Purchase #5 $73.36   10.149%   $80.8053   $80.81   $80.80   $80.80  
  Purchase #6 $197.37   11.432%   $219.9333   $219.93   $219.95   $219.90  
  Purchase #7 $63.55   4.147%   $66.1854   $66.19   $66.20   $66.20  
  7 purchase totals = $773.45       $830.5826   $830.59   $830.60   $830.50