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 $236.82   7.816%   $255.3299   $255.33   $255.35   $255.30  
  Purchase #2 $204.62   12.268%   $229.7228   $229.72   $229.70   $229.70  
  Purchase #3 $197.35   3.243%   $203.7501   $203.75   $203.75   $203.80  
  Purchase #4 $233.39   9.174%   $254.8012   $254.80   $254.80   $254.80  
  Purchase #5 $51.83   8.451%   $56.2102   $56.21   $56.20   $56.20  
  Purchase #6 $225.55   8.731%   $245.2428   $245.24   $245.25   $245.20  
  Purchase #7 $248.38   4.460%   $259.4577   $259.46   $259.45   $259.50  
  7 purchase totals = $1,397.94       $1,504.5147   $1,504.51   $1,504.50   $1,504.50