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 $224.91   12.739%   $253.5613   $253.56   $253.55   $253.60  
  Purchase #2 $88.58   6.155%   $94.0321   $94.03   $94.05   $94.00  
  Purchase #3 $179.59   13.671%   $204.1417   $204.14   $204.15   $204.10  
  Purchase #4 $44.72   6.471%   $47.6138   $47.61   $47.60   $47.60  
  Purchase #5 $185.93   12.828%   $209.7811   $209.78   $209.80   $209.80  
  Purchase #6 $93.57   9.563%   $102.5181   $102.52   $102.50   $102.50  
  Purchase #7 $21.64   4.582%   $22.6315   $22.63   $22.65   $22.60  
  7 purchase totals = $838.94       $934.2796   $934.27   $934.30   $934.20