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 $18.36   5.155%   $19.3065   $19.31   $19.30   $19.30  
  Purchase #2 $20.35   3.593%   $21.0812   $21.08   $21.10   $21.10  
  Purchase #3 $216.86   8.561%   $235.4254   $235.43   $235.45   $235.40  
  Purchase #4 $9.38   7.489%   $10.0825   $10.08   $10.10   $10.10  
  Purchase #5 $174.29   5.690%   $184.2071   $184.21   $184.20   $184.20  
  Purchase #6 $28.72   10.710%   $31.7959   $31.80   $31.80   $31.80  
  Purchase #7 $17.90   4.644%   $18.7313   $18.73   $18.75   $18.70  
  7 purchase totals = $485.86       $520.6299   $520.64   $520.70   $520.60