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 $21.85   4.803%   $22.8995   $22.90   $22.90   $22.90  
  Purchase #2 $13.47   13.132%   $15.2389   $15.24   $15.25   $15.20  
  Purchase #3 $139.91   6.527%   $149.0419   $149.04   $149.05   $149.00  
  Purchase #4 $179.34   13.652%   $203.8235   $203.82   $203.80   $203.80  
  Purchase #5 $175.77   13.547%   $199.5816   $199.58   $199.60   $199.60  
  Purchase #6 $163.58   3.502%   $169.3086   $169.31   $169.30   $169.30  
  Purchase #7 $26.97   12.814%   $30.4259   $30.43   $30.45   $30.40  
  7 purchase totals = $720.89       $790.3199   $790.32   $790.35   $790.20