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 $9.78   10.365%   $10.7937   $10.79   $10.80   $10.80  
  Purchase #2 $246.57   11.261%   $274.3362   $274.34   $274.35   $274.30  
  Purchase #3 $39.36   4.330%   $41.0643   $41.06   $41.05   $41.10  
  Purchase #4 $62.82   11.358%   $69.9551   $69.96   $69.95   $70.00  
  Purchase #5 $8.63   6.681%   $9.2066   $9.21   $9.20   $9.20  
  Purchase #6 $220.67   8.248%   $238.8709   $238.87   $238.85   $238.90  
  Purchase #7 $174.87   8.632%   $189.9648   $189.96   $189.95   $190.00  
  7 purchase totals = $762.70       $834.1916   $834.19   $834.15   $834.30