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 $18.86   9.474%   $20.6468   $20.65   $20.65   $20.60  
  Purchase #2 $127.80   4.390%   $133.4104   $133.41   $133.40   $133.40  
  Purchase #3 $47.97   12.444%   $53.9394   $53.94   $53.95   $53.90  
  Purchase #4 $172.46   6.299%   $183.3233   $183.32   $183.30   $183.30  
  Purchase #5 $101.25   9.579%   $110.9487   $110.95   $110.95   $110.90  
  Purchase #6 $7.64   4.667%   $7.9966   $8.00   $8.00   $8.00  
  Purchase #7 $150.79   7.684%   $162.3767   $162.38   $162.40   $162.40  
  7 purchase totals = $626.77       $672.6419   $672.65   $672.65   $672.50