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 $239.94   12.810%   $270.6763   $270.68   $270.70   $270.70  
  Purchase #2 $63.48   9.140%   $69.2821   $69.28   $69.30   $69.30  
  Purchase #3 $150.54   11.605%   $168.0102   $168.01   $168.00   $168.00  
  Purchase #4 $60.54   11.471%   $67.4845   $67.48   $67.50   $67.50  
  Purchase #5 $231.62   12.474%   $260.5123   $260.51   $260.50   $260.50  
  Purchase #6 $113.87   9.180%   $124.3233   $124.32   $124.30   $124.30  
  Purchase #7 $56.35   12.722%   $63.5188   $63.52   $63.50   $63.50  
  7 purchase totals = $916.34       $1,023.8075   $1,023.80   $1,023.80   $1,023.80