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 $28.47   9.225%   $31.0964   $31.10   $31.10   $31.10  
  Purchase #2 $20.81   11.709%   $23.2466   $23.25   $23.25   $23.20  
  Purchase #3 $62.87   11.509%   $70.1057   $70.11   $70.10   $70.10  
  Purchase #4 $132.25   8.717%   $143.7782   $143.78   $143.80   $143.80  
  Purchase #5 $191.50   10.596%   $211.7913   $211.79   $211.80   $211.80  
  Purchase #6 $164.45   11.508%   $183.3749   $183.37   $183.35   $183.40  
  Purchase #7 $155.27   4.139%   $161.6966   $161.70   $161.70   $161.70  
  7 purchase totals = $755.62       $825.0897   $825.10   $825.10   $825.10