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 $245.57   13.205%   $277.9975   $278.00   $278.00   $278.00  
  Purchase #2 $139.51   7.120%   $149.4431   $149.44   $149.45   $149.40  
  Purchase #3 $61.40   4.398%   $64.1004   $64.10   $64.10   $64.10  
  Purchase #4 $194.67   13.540%   $221.0283   $221.03   $221.05   $221.00  
  Purchase #5 $62.55   4.312%   $65.2472   $65.25   $65.25   $65.20  
  Purchase #6 $83.65   12.231%   $93.8812   $93.88   $93.90   $93.90  
  Purchase #7 $176.31   6.768%   $188.2427   $188.24   $188.25   $188.20  
  7 purchase totals = $963.66       $1,059.9404   $1,059.94   $1,060.00   $1,059.80