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 $28.69   9.392%   $31.3846   $31.38   $31.40   $31.40  
  Purchase #2 $244.30   12.323%   $274.4051   $274.41   $274.40   $274.40  
  Purchase #3 $129.51   8.454%   $140.4588   $140.46   $140.45   $140.50  
  Purchase #4 $110.65   13.340%   $125.4107   $125.41   $125.40   $125.40  
  Purchase #5 $96.60   6.574%   $102.9505   $102.95   $102.95   $103.00  
  Purchase #6 $78.67   12.812%   $88.7492   $88.75   $88.75   $88.70  
  Purchase #7 $121.58   3.194%   $125.4633   $125.46   $125.45   $125.50  
  7 purchase totals = $810.00       $888.8222   $888.82   $888.80   $888.90