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 $7.70   5.140%   $8.0958   $8.10   $8.10   $8.10  
  Purchase #2 $19.77   6.419%   $21.039   $21.04   $21.05   $21.00  
  Purchase #3 $145.85   7.792%   $157.2146   $157.21   $157.20   $157.20  
  Purchase #4 $29.36   11.453%   $32.7226   $32.72   $32.70   $32.70  
  Purchase #5 $24.99   7.874%   $26.9577   $26.96   $26.95   $27.00  
  Purchase #6 $80.95   6.124%   $85.9074   $85.91   $85.90   $85.90  
  Purchase #7 $90.70   11.568%   $101.1922   $101.19   $101.20   $101.20  
  7 purchase totals = $399.32       $433.1293   $433.13   $433.10   $433.10