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 $44.95   12.131%   $50.4029   $50.40   $50.40   $50.40  
  Purchase #2 $160.34   11.869%   $179.3708   $179.37   $179.35   $179.40  
  Purchase #3 $212.75   6.222%   $225.9873   $225.99   $226.00   $226.00  
  Purchase #4 $127.96   13.142%   $144.7765   $144.78   $144.80   $144.80  
  Purchase #5 $121.53   9.388%   $132.9392   $132.94   $132.95   $132.90  
  Purchase #6 $100.49   9.757%   $110.2948   $110.29   $110.30   $110.30  
  Purchase #7 $49.85   8.319%   $53.997   $54.00   $54.00   $54.00  
  7 purchase totals = $817.87       $897.7685   $897.77   $897.80   $897.80