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 $49.95   5.133%   $52.5139   $52.51   $52.50   $52.50  
  Purchase #2 $224.78   11.327%   $250.2408   $250.24   $250.25   $250.20  
  Purchase #3 $119.66   13.651%   $135.9948   $135.99   $136.00   $136.00  
  Purchase #4 $43.83   4.862%   $45.961   $45.96   $45.95   $46.00  
  Purchase #5 $205.91   12.581%   $231.8155   $231.82   $231.80   $231.80  
  Purchase #6 $27.47   4.203%   $28.6246   $28.62   $28.60   $28.60  
  Purchase #7 $67.27   7.199%   $72.1128   $72.11   $72.10   $72.10  
  7 purchase totals = $738.87       $817.2634   $817.25   $817.20   $817.20