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 $100.35   8.752%   $109.1326   $109.13   $109.15   $109.10  
  Purchase #2 $109.98   3.764%   $114.1196   $114.12   $114.10   $114.10  
  Purchase #3 $138.63   10.663%   $153.4121   $153.41   $153.40   $153.40  
  Purchase #4 $78.91   12.426%   $88.7154   $88.72   $88.70   $88.70  
  Purchase #5 $154.55   10.538%   $170.8365   $170.84   $170.85   $170.80  
  Purchase #6 $92.64   6.423%   $98.5903   $98.59   $98.60   $98.60  
  Purchase #7 $75.73   9.739%   $83.1053   $83.11   $83.10   $83.10  
  7 purchase totals = $750.79       $817.9118   $817.92   $817.90   $817.80