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 $12.99   6.346%   $13.8143   $13.81   $13.80   $13.80  
  Purchase #2 $158.60   8.599%   $172.238   $172.24   $172.25   $172.20  
  Purchase #3 $199.47   4.118%   $207.6842   $207.68   $207.70   $207.70  
  Purchase #4 $186.25   9.776%   $204.4578   $204.46   $204.45   $204.50  
  Purchase #5 $163.31   3.828%   $169.5615   $169.56   $169.55   $169.60  
  Purchase #6 $179.73   12.224%   $201.7002   $201.70   $201.70   $201.70  
  Purchase #7 $99.58   8.797%   $108.3401   $108.34   $108.35   $108.30  
  7 purchase totals = $999.93       $1,077.7961   $1,077.79   $1,077.80   $1,077.80