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 $21.75   12.259%   $24.4163   $24.42   $24.40   $24.40  
  Purchase #2 $66.99   10.475%   $74.0072   $74.01   $74.00   $74.00  
  Purchase #3 $71.82   8.401%   $77.8536   $77.85   $77.85   $77.90  
  Purchase #4 $38.29   10.643%   $42.3652   $42.37   $42.35   $42.40  
  Purchase #5 $138.27   10.451%   $152.7206   $152.72   $152.70   $152.70  
  Purchase #6 $142.54   7.865%   $153.7508   $153.75   $153.75   $153.80  
  Purchase #7 $80.70   9.819%   $88.6239   $88.62   $88.60   $88.60  
  7 purchase totals = $560.36       $613.7376   $613.74   $613.65   $613.80