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 $238.76   8.688%   $259.5035   $259.50   $259.50   $259.50  
  Purchase #2 $167.79   4.198%   $174.8338   $174.83   $174.85   $174.80  
  Purchase #3 $51.31   4.269%   $53.5004   $53.50   $53.50   $53.50  
  Purchase #4 $245.75   4.560%   $256.9562   $256.96   $256.95   $257.00  
  Purchase #5 $60.57   4.786%   $63.4689   $63.47   $63.45   $63.50  
  Purchase #6 $219.35   10.270%   $241.8772   $241.88   $241.90   $241.90  
  Purchase #7 $118.67   7.119%   $127.1181   $127.12   $127.10   $127.10  
  7 purchase totals = $1,102.20       $1,177.2581   $1,177.26   $1,177.25   $1,177.30