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 $81.63   12.261%   $91.6387   $91.64   $91.65   $91.60  
  Purchase #2 $157.44   6.221%   $167.2343   $167.23   $167.25   $167.20  
  Purchase #3 $130.64   6.156%   $138.6822   $138.68   $138.70   $138.70  
  Purchase #4 $92.28   3.578%   $95.5818   $95.58   $95.60   $95.60  
  Purchase #5 $46.73   7.854%   $50.4002   $50.40   $50.40   $50.40  
  Purchase #6 $199.31   3.132%   $205.5524   $205.55   $205.55   $205.60  
  Purchase #7 $82.94   9.478%   $90.8011   $90.80   $90.80   $90.80  
  7 purchase totals = $790.97       $839.8907   $839.88   $839.95   $839.90