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 $75.25   6.867%   $80.4174   $80.42   $80.40   $80.40  
  Purchase #2 $42.25   8.502%   $45.8421   $45.84   $45.85   $45.80  
  Purchase #3 $23.88   13.230%   $27.0393   $27.04   $27.05   $27.00  
  Purchase #4 $174.86   8.536%   $189.786   $189.79   $189.80   $189.80  
  Purchase #5 $35.36   10.729%   $39.1538   $39.15   $39.15   $39.20  
  Purchase #6 $64.30   3.214%   $66.3666   $66.37   $66.35   $66.40  
  Purchase #7 $227.42   3.459%   $235.2865   $235.29   $235.30   $235.30  
  7 purchase totals = $643.32       $683.8917   $683.90   $683.90   $683.90