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 $75.98   7.175%   $81.4316   $81.43   $81.45   $81.40  
  Purchase #2 $117.25   10.549%   $129.6187   $129.62   $129.60   $129.60  
  Purchase #3 $74.93   7.199%   $80.3242   $80.32   $80.30   $80.30  
  Purchase #4 $132.65   9.329%   $145.0249   $145.02   $145.00   $145.00  
  Purchase #5 $34.88   11.428%   $38.8661   $38.87   $38.85   $38.90  
  Purchase #6 $132.43   4.177%   $137.9616   $137.96   $137.95   $138.00  
  Purchase #7 $77.44   11.823%   $86.5957   $86.60   $86.60   $86.60  
  7 purchase totals = $645.56       $699.8228   $699.82   $699.75   $699.80