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 $37.99   4.815%   $39.8192   $39.82   $39.80   $39.80  
  Purchase #2 $5.74   7.783%   $6.1867   $6.19   $6.20   $6.20  
  Purchase #3 $211.36   4.358%   $220.5711   $220.57   $220.55   $220.60  
  Purchase #4 $104.85   8.505%   $113.7675   $113.77   $113.75   $113.80  
  Purchase #5 $48.78   10.636%   $53.9682   $53.97   $53.95   $54.00  
  Purchase #6 $102.90   6.220%   $109.3004   $109.30   $109.30   $109.30  
  Purchase #7 $124.66   12.439%   $140.1665   $140.17   $140.15   $140.20  
  7 purchase totals = $636.28       $683.7796   $683.79   $683.70   $683.90