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 $131.54   5.619%   $138.9312   $138.93   $138.95   $138.90  
  Purchase #2 $205.85   7.303%   $220.8832   $220.88   $220.90   $220.90  
  Purchase #3 $201.70   8.408%   $218.6589   $218.66   $218.65   $218.70  
  Purchase #4 $215.52   4.486%   $225.1882   $225.19   $225.20   $225.20  
  Purchase #5 $144.45   12.332%   $162.2636   $162.26   $162.25   $162.30  
  Purchase #6 $169.90   6.385%   $180.7481   $180.75   $180.75   $180.70  
  Purchase #7 $9.47   11.252%   $10.5356   $10.54   $10.55   $10.50  
  7 purchase totals = $1,078.43       $1,157.2088   $1,157.21   $1,157.25   $1,157.20