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 $124.38   3.539%   $128.7818   $128.78   $128.80   $128.80  
  Purchase #2 $241.33   4.637%   $252.5205   $252.52   $252.50   $252.50  
  Purchase #3 $162.98   12.620%   $183.5481   $183.55   $183.55   $183.50  
  Purchase #4 $53.26   7.430%   $57.2172   $57.22   $57.20   $57.20  
  Purchase #5 $183.98   8.622%   $199.8428   $199.84   $199.85   $199.80  
  Purchase #6 $78.47   9.175%   $85.6696   $85.67   $85.65   $85.70  
  Purchase #7 $28.31   12.767%   $31.9243   $31.92   $31.90   $31.90  
  7 purchase totals = $872.71       $939.5043   $939.50   $939.45   $939.40