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 $34.93   12.609%   $39.3343   $39.33   $39.35   $39.30  
  Purchase #2 $85.84   6.841%   $91.7123   $91.71   $91.70   $91.70  
  Purchase #3 $163.95   6.445%   $174.5166   $174.52   $174.50   $174.50  
  Purchase #4 $213.32   7.532%   $229.3873   $229.39   $229.40   $229.40  
  Purchase #5 $138.33   7.354%   $148.5028   $148.50   $148.50   $148.50  
  Purchase #6 $101.67   12.683%   $114.5648   $114.56   $114.55   $114.60  
  Purchase #7 $207.74   8.392%   $225.1735   $225.17   $225.15   $225.20  
  7 purchase totals = $945.78       $1,023.1916   $1,023.18   $1,023.15   $1,023.20