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 $81.70   3.294%   $84.3912   $84.39   $84.40   $84.40  
  Purchase #2 $123.48   7.755%   $133.0559   $133.06   $133.05   $133.10  
  Purchase #3 $6.95   12.850%   $7.8431   $7.84   $7.85   $7.80  
  Purchase #4 $139.52   11.440%   $155.4811   $155.48   $155.50   $155.50  
  Purchase #5 $65.48   13.511%   $74.327   $74.33   $74.35   $74.30  
  Purchase #6 $12.64   7.659%   $13.6081   $13.61   $13.60   $13.60  
  Purchase #7 $80.60   8.555%   $87.4953   $87.50   $87.50   $87.50  
  7 purchase totals = $510.37       $556.2017   $556.21   $556.25   $556.20