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 $57.94   12.501%   $65.1831   $65.18   $65.20   $65.20  
  Purchase #2 $28.43   8.234%   $30.7709   $30.77   $30.75   $30.80  
  Purchase #3 $87.56   9.552%   $95.9237   $95.92   $95.90   $95.90  
  Purchase #4 $170.60   10.168%   $187.9466   $187.95   $187.95   $187.90  
  Purchase #5 $241.75   13.201%   $273.6634   $273.66   $273.65   $273.70  
  Purchase #6 $170.65   13.861%   $194.3038   $194.30   $194.30   $194.30  
  Purchase #7 $8.33   13.135%   $9.4241   $9.42   $9.40   $9.40  
  7 purchase totals = $765.26       $857.2156   $857.20   $857.15   $857.20