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 $110.70   12.668%   $124.7235   $124.72   $124.70   $124.70  
  Purchase #2 $240.77   6.462%   $256.3286   $256.33   $256.35   $256.30  
  Purchase #3 $120.80   6.540%   $128.7003   $128.70   $128.70   $128.70  
  Purchase #4 $17.54   7.444%   $18.8457   $18.85   $18.85   $18.80  
  Purchase #5 $45.73   12.401%   $51.401   $51.40   $51.40   $51.40  
  Purchase #6 $50.27   10.446%   $55.5212   $55.52   $55.50   $55.50  
  Purchase #7 $148.33   5.293%   $156.1811   $156.18   $156.20   $156.20  
  7 purchase totals = $734.14       $791.7014   $791.70   $791.70   $791.60