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 $51.48   11.459%   $57.3791   $57.38   $57.40   $57.40  
  Purchase #2 $17.73   12.735%   $19.9879   $19.99   $20.00   $20.00  
  Purchase #3 $150.95   12.574%   $169.9305   $169.93   $169.95   $169.90  
  Purchase #4 $100.90   8.227%   $109.201   $109.20   $109.20   $109.20  
  Purchase #5 $12.44   10.500%   $13.7462   $13.75   $13.75   $13.70  
  Purchase #6 $243.43   3.456%   $251.8429   $251.84   $251.85   $251.80  
  Purchase #7 $142.67   4.706%   $149.3841   $149.38   $149.40   $149.40  
  7 purchase totals = $719.60       $771.4717   $771.47   $771.55   $771.40