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 $48.52   5.163%   $51.0251   $51.03   $51.05   $51.00  
  Purchase #2 $226.54   6.745%   $241.8201   $241.82   $241.80   $241.80  
  Purchase #3 $20.43   11.661%   $22.8123   $22.81   $22.80   $22.80  
  Purchase #4 $236.26   13.223%   $267.5007   $267.50   $267.50   $267.50  
  Purchase #5 $242.39   10.718%   $268.3694   $268.37   $268.35   $268.40  
  Purchase #6 $33.46   6.759%   $35.7216   $35.72   $35.70   $35.70  
  Purchase #7 $29.97   3.558%   $31.0363   $31.04   $31.05   $31.00  
  7 purchase totals = $837.57       $918.2855   $918.29   $918.25   $918.20